A/N: SO WHERE DID THIS COME FROM! well allow me to tell you that i accidentally remembered that i used to watch this show and then i started reading fanfic and then... this... just came out of nowhere? i am not in this fandom! i haven't watched this show since the 5th grade so nothing is probably accurate! oh my god what have i done this is a monster i am so sorry i don't know. i don't know how to write these characters. please forgive me.

Okay – action.

The opening scene is pretty sloppy. No one took too long thinking about the font of the title or when to work in the actor names. Okay, now zoom in – Derek Venturi in college! The title of the movie is like, University of Derek or some other low-budget, unoriginal, uncreative name. But he doesn't mind so much. His favorite part is coming up, which is, of course, his first scene.

And – cut. University of Derek.

…

The car ride is unbearable. He keeps looking for things to strangle himself with. The seatbelt, while a good choice, is too tight to get any sort of function out of it. He'd have to waste a lot of time untying his shoe laces, and –

"Der-ek!"

"What."

Casey's knuckles are white on the steering wheel. She is looking straight ahead, trying not to fail her first I Am College Bound test, which is undoubtedly Please Don't Crash the Car. But she's furious. Of course.

"You're not even listening to me," she huffs. "I'm trying to set clear ground rules here, and –"

"—and I'm making it oh-so-difficult. Why do we need ground rules?"

Casey glances at him incredulously and continues a little manically, "What if I make friends and you try to date them? What if you date them and you hurt them or what if, even worse, you date them and you like them and then I have to watch you marry one of my friends? Or what if…"

The possible situations are endless. What if Derek deliberately makes friends with her friends and then unceremoniously screws them and even more unceremoniously dumps them and what if she's at her dorm when it happens? What if she has to listen to him doing "it" with her friends? The list goes on. Derek tries not to fall asleep.

"What if," Casey continues, her voice growing hoarse, "your power goes out and you try to come to my dorm to stay the night – which is never, ever , ever, ever, ever –"

Derek pinches the bridge of his nose. This would normally be a great time to argue but he's really just worried that she's going to crash the car, and that would be the worst way to start off his college experience. Like, dead.

"—ever , ever going to happen. Ever. By the way." Casey takes a breath. "So. We need –"

"—Ground rules."

"Yes." She sighs with relief. "So, rule number one…"

…

Rule #1. Derek Venturi cannot date one of Casey MacDonald's friends. Period. Not even if they come onto him first. ("What if I don't know if they're your friends?") By this clause, he must check if they are friends of hers before dating them, or sleeping with them, or chatting them up at a party. Period.

Rule #2. Derek Venturi is not allowed in Casey MacDonald's dorm. Ever. (Ever. Ever. Ever.) Not even if his power goes out. Not even if his roommates kick him out. Not even if he's invited there for a party. ("No one is going to throw a party at your dorm, Casey. Get real.") Not ever. Special occasions may include a family tragedy or death or coma. But even then.

Rule #3. Periodic check-ins are encouraged but not required. They're not actually encouraged, either. Actually, no. No periodic check-ins.

Rule #4. No drunk-dialing. No butt-dialing. No accidental-but-on-purpose-dialing to purposefully embarrass her. No calling at all. Unless of course, a family tragedy or death or coma. (But even then.)

Rule #5. Visiting home once every three months is required but not encouraged to do it together. ("It would save gas, Derek.") Not even because of that. Unless of course, a family tragedy, or death, or coma. ("But even then.")

Rule #6. These rules are not subject to change. They will not change. We will have a good year. Without each other. Period.

…

He breaks the first rule almost immediately and it was almost entirely on accident.

So Derek's new roommate turns out to be completely and utterly awesome on all accounts – his name is Douglas [Something] the Third, and okay, Derek didn't listen that much but he thought it was totally cool that his roommate is the Third anything. Like, Douglas was such an important name that is family needed to renew it every generation. That was pretty tops.

Douglas happens to be loaded and lazy. That's also pretty tops. Which means Douglas is everything that Derek needs in a roommate – connected with the people, popular, always ready to get drunk and laid and not in the exact same order – plus completely and conveniently rich. Derek is happy. Derek is happier than he's ever been in a really long time and that's the best part of this whole thing, that he doesn't have to worry about Casey showing up and ruining it at all.

Derek forgets the part where he's supposed to ask every girl he meets if, by the way, are you a roommate to one very special Casey MacDonald? So Derek meets this girl who introduces herself as Katie, and she has blonde hair that was dip-dyed red, and he's not one-hundred-percent into it but he's a little bit drunk and she's a little bit easy, and she asks him if he wants to take this conversation somewhere else.

It's only until after, when he's zipping up his jeans and tugging back on his sweater, that she glances at the clock and lets out a mournful sigh. "Jesus Christ. My roommate is going to slaughter me."

By now the buzz is gone but Derek is still feeling good. He gives a sloppy, lopsided grin. "Did you get stuck with your mom as a roommate or something?"

In the better lighting, Derek notices that she's pretty, a lot prettier than she looked back there, which is a nice surprise. She rolls her eyes. "Almost. Close enough. I got stuck with this crazy, neurotic, should-be-admitted-'cause-she's-clinically-insane girl. And I was supposed to be back –" She checks the clock again and laughs. "Like, two hours ago. For a let's-get-to-know-each-other powwow with each other."

"Shit, sounds like you missed a party."

"Sounds like they did."

They both laugh and Derek likes this a lot. This is the easiest it's been in a while. He can really get used to Douglas and college and girls with dip-dyed red hair. He really could.

After a few seconds of less-than-awkward idling, Katie extends her hand in a shake and says, "Nice to meet you, Derek. Let's do this again sometime."

She says it like this was a business meeting or a formal date or some kind of casually-arranged affair, and he likes that so much that he can't help but fall a little bit in like with her. "Give me your dorm room number and maybe I'll stop by sometime."

Katie laughs at that. "Not even my phone-number?"

"Too cliché."

She seems to agree with that and writes it on the inside of his wrist with a worn-down eyeliner pencil. "Don't forget," she says. "I will be expecting you. Maybe you can meet my roommate-slash-mom, too. Then you'll know I wasn't just making her up."

"Will do, Ms. Katie."

And he doesn't even know it yet but he's totally broken the first rule. It's not like it was deliberate. How was he supposed to know?

…

"How was I supposed to know?"

"Rule #4. No drunk-dialing. No butt-dialing. No accidental-but-on-purpose-dialing to purposefully embarrass her. No calling at all. Unless of course, a family tragedy or death or coma. (But even then.)"

They broke that rule, too. But Casey waves it off and says that this counts as a family tragedy. "You slept with my roommate and it's only been three days!" Her voice is a shriek at this point and he has to hold the phone away from his ear about three inches.

"How was I supposed to know?" he repeats. "Was I supposed to ask her? I didn't even think about it –"

"—and why didn't you? That was the deal we –"

"—because that's so stupid, why should I –"

"—because that was the deal!"

Douglas pokes his head in the doorway and raises an eyebrow quizzically. He mouths, "Is that your mom?"

Derek shakes his head, a headache already starting to form behind his eyes.

"… and I just thought you would have a little bit more consideration because, up until now, which is totally justified, I haven't broken any of the rules."

Derek wants to hang up oh-god-so-badly, but he did promise to be amicable and considerate, even if those words are not in his vocabulary and he kind of had to look up the meaning when she said them. He puts on his best patronizing-parent voice and says, "Well, if it's such an inconvenience, I won't see Katie anymore."

The shriek of protest is even louder this time. "No!" she squeals. "Now you have to see her. You have to be nice to her. You have to be good to her until she comes to her senses."

"That doesn't make any –"

"End of discussion, Derek. Please. I am asking you… nicely. Please."

And that's how Casey set Derek up on his first official date with Katie.

…

Rule #7. These rules are sometimes subject to change, but only rarely.

…

Derek is still not allowed to enter Casey's dorm/torture chamber, and apparently that rule also extends to the hallway outside of her dorm and the elevator leading up to said hallway, so Derek waits for Katie downstairs. He ends up true to his word about being a nice guy, takes her out to dinner, a movie, the works.

It feels… oddly easy.

Not in the trained, obedient dog way, but in the casual, nice, totally stress-free way. Katie makes it clear that she's not looking for a white picket fence or anything and the part of Derek's brain that shuts down at the word "commitment" springs to life and does a jig. So he finds this part easy.

And, okay, she's totally an eight-point-five if he's not being generous, so that's a plus. And she's funny, and not a total bummer, so he finds this part easy too.

Except, Casey calls Katie halfway through to make sure that she hasn't been dumped or hurt or harmed in any way, physically/emotionally/mentally. Katie says a lot of "uh-huh" and "no, Casey" and "I know, Casey" and makes a face at Derek like oh-god-someone-help-me. He finds that easy, too.

Everything feels easy and Derek starts to resent that, towards the end, just a little bit.

…

"Are you sure?" he repeats for the third or fourth time.

Katie unlocks the door quietly, rolling her eyes at him like, duh, and he feels like an idiot for asking. "Casey is asleep by now. She's probably been asleep since like, seven. Classes start tomorrow." At the reminder Derek can feel himself deflate. Right, yeah. Classes. College. That's why we're here.

Derek still feels tentative, because he doesn't want to deal with a stressed-out Casey this close to classes, but a part of him delights in the idea of breaking her biggest, most important rule. At least, he considers it the most important rule – seeing each other. On the same turf. In the same dorm.

He would love that. He thinks about what her face would look like, all etched surprise and anger and indignation, even while Katie is kissing him.

And as these things sort of go, he doesn't see Casey. She probably really did go to sleep at seven, so she can wake up at six all bright-eyed and ready for the day, beauty-rest completed (and failed), notes written and categorized even before there were notes to take. Because that's who Casey is. She embodies college.

And this –

This is Derek.

…

Derek doesn't see Casey for the first few months and he finds that utterly ridiculous, because he's been dating her roommate for the entire time and the only time they've communicated is through Katie, like five-year-olds on non-speaking terms.

"Katie, tell Casey that Nora is really freaking out about her not calling in a few days."

"Casey says to tell you that she hasn't called because she's been busy, and your life isn't her business anyway. She also says that if she ever finds out you've spent the night, she'll castrate you."

And so on.

Derek finds it… completely crazy, honestly. He doesn't miss her because being without her for months a time is some kind of proof of a benevolent higher power, but he still expected something. He expected –

What did he expect? He doesn't really know and that makes him angry, and he curses her name twelve times, says her name in that way that makes it a swear. Casey. Casey-fucking-MacDonald.

…

When Derek does see Casey, it's at a party and he's drunk and she looks tense. He thinks to laugh at her but then he feels kind of bad for her, and then he doesn't because what the fuck? And then he thinks, wow, I'm drunk, and then he thinks, what the fuck is Casey doing here?

He approaches her from behind and touches the small of her back in a really creepy way, making her jump out of her skin and let out some kind of half-strangled yelp. He relishes in the look on her face as she turns to meet him. Her widened eyes narrow to slits and she hits him once, hard.

"Der-ek!"

(Okay, and that – that makes his stomach settle, makes him feel like everything is a little more normal. He – kind of missed… that.)

"Hey, princess." His grin is wide enough to seem fake but he's almost so happy to see her, and so fucking happy at the look on her face when she sees him, that everything just feels better. She looks like she's going to kill him and he loves it. This is what he missed. Not her, just the thrill of making her hate him.

She sniffs almost immediately, pretty haughtily he might add, which, okay, princess. "You're drunk."

"Wow," Derek says, putting a hand to his heart. "I'm so touched you did that psychoanalysis completely free of charge. That was really generous. Thank you."

Of course, she hits him again, and he likes that, too. "Where have you been?"

Derek has half a mind to hit her back, because, what?

"I've been… here. I've been with Katie. I've been dating your roommate for like three months, Case." For some reason the way she asked him made it sound like she was blaming him for something and he couldn't imagine why she would be doing that. She can't be angry that she hasn't seen him because she's made it pretty clear that seeing him would ruin her college experience.

Casey sips delicately on something that looks like beer but smells like ginger ale. "I've been trying not to remember," she says, face scrunching up in some awful grimace.

"Have you?"

And just like, all of a sudden, Derek gets really fucking mad.

Because, okay, fuck this – college isn't code word for "a totally different planet." Yeah, maybe he and Casey came from the same house and she doesn't like to admit it, and he doesn't like to think about it too much, but that doesn't mean – God, that didn't mean they weren't –

It feels like betrayal of some kind.

The anger sort of rises out of nowhere and starts to fizzle out in his brain, like it was never there. Why hasn't he seen her for three months? Why has he tried to follow those stupid ground rules? Why were there ever ground rules?

Casey looks at him for a second and doesn't say anything. Like, what the fuck? Was she seriously asking him where he's been, like she doesn't already know? Like, she doesn't know where his dorm is? She could have visited. He could have. What the fuck. What the fuck.

And then just like that the anger is gone and all that's left is –

"Derek!"

Katie leaps into his arms, smelling like an ashtray married a beer keg, and kisses him full on the lips. He can hear Casey groan and shuffle away and when he looks up, gasping for air, she's already gone.

What the fuck.

…

Derek's brain for the next few daysis sort of on overdrive.

He thinks about Casey for inordinate amounts of time, and at weird moments, like when Katie comes over and fucks him on the couch and he thinks about whether Casey knows what he's doing right now. And that makes him angry, and it makes him more vengeful, so he fucks Katie harder, and, well –

He's never really gotten to the bottom of the way his brain works.

He wants to see Casey but he also wants to punch her in the face, if he were the kind of dude that hit girls, which he wasn't because, duh. He wants to see her but he also wishes she were dead or something. He doesn't understand why ground rules ever needed to exist and it makes him hate her.

It's not like they never had anything in common.

She could at least – call.

And why is she showing up at these random parties? How does she even manage that? Who even invites her anywhere?

He decides, fuck the rules and tells Katie to invite him over more often, and she arches a perfectly-shaped eyebrow and laughs at him like he's an idiot, and then he tells her he's serious about it, and she gets really excited for some reason. Like her dream has been to actually fuck him on her own bed. Or something.

It's getting easier but because it's getting easier he kind of hates it more. But he doesn't think about what that means.

…

They go (unfortunately) unnoticed on their first couple trysts at Katie's dorm, but like all things do, they kind of had to succumb to some sort of gravity. This time, his gravity is Casey, and he discovers it on an extremely early Thursday morning whilst sort of buzzed, on his way out of the dorm.

"Derek?"

Well, he almost made it, but this is kind of better.

"Sister dearest!" Derek opens his arms welcomingly, but Casey just stares at him as if she's seeing a ghost. Like, she doesn't even remember she had a brother. (Step-brother. Same difference.)

"What are you –" and slow, easy recognition sort of drips down her face, and then she makes some kind of terrible noise, quiet enough not to wake anyone else, and drops her head into her hands. "God, I thought we had –"

"Ground rules?" Derek can feel himself getting angry again, bubbling over with the words he doesn't say. He knows he has to somehow keep a cap on that but, honestly –

"Can't you just… find somewhere else to go?"

He thinks about it, and then very quickly decides, "Nope."

Casey folds her arms over her chest and he notices, sort of out of nowhere, that she's wearing the pinkest, fluffiest bathrobe he's ever seen, and the familiarity of it all makes him so goddamn pleased. Like this is how it should always be. He always needs a Casey around to yell at and banter with.

"But… Der-ek!" she stage-whispers. "I don't want to hear you guys… you know… my room is right – it's right –"

"Were you gonna listen?"

A sort of weak thrill manifests itself in his stomach, where he can feel something angry and happy and excited take root. Casey flushes the same color of red as Katie's dip-dye, and starts trying to stutter out something intelligible.

"No – I would never – God, Derek, that's so – like I would ever –"

"It's okay if you're interested. Just didn't ever think you'd admit –"

"I am not interested in –"

"Don't you think it's kind of a little too… Flowers in the Attic?"

His skin is like jumping with how fucking happy he is that this is even happening, and, yeah, okay, he needs some kind of therapy, but this is enough for now. Casey looks like she's vibrating with absolute frustration and it makes him wish he could capture this moment for-fucking-ever. He never wants to let this go.

Casey tries to say something and Derek says, "Look, Case, I'm really flattered by your interest –"

"Oh my God –"

"—but it just wouldn't be fair to Katie."

It's too early in the morning for Casey to explode the way she obviously wants to, and Derek relishes in that even more. It's like – okay, he missed this. He missed having her around if only for the part where he gets to make her wish that she wasn't. He didn't actually ever mean for her to leave. Forever.

Casey ushers him out quickly and quietly and then stands there in her bathrobe, trying to make him leave. He wonders then if she's ever really thought about –

And then he shuts that thought down completely, doesn't even –

"You're sick, Derek."

Somewhere, deep down, really quite deep down, he knows that he is.

…

Exams come around, which means people are having weird, crazy parties for the timing. Derek thinks that people are just kind of hysterical about exams and panic needs company, so panic invites sixty people over and gets really drunk and then panic fucks like, three people at once and then cries about it later.

As it so happens, Derek is not panicking, so he's not having a bad time. It's all just kind of okay. Katie isn't panicking either, so she's talking to a guy across the room and looking really into it, and Derek just doesn't care, so he doesn't get mad, and he just sits there.

Things sort of happen, but they don't happen to him.

The dorm clears out pretty early, and Derek feels like it's way too empty for him to even be there. It's that scary kind of empty, where he knows that there are going to be a few vultures coming by to pick off the drunk girls who won't say no, and he doesn't want to get caught up in it, so he moves to leave. He's not even buzzed. Everything feels –

He doesn't really even feel anything.

And then he does.

Because, as Derek is shoving out into the hallway, he notices the fucking queen of panic pushed flat against a wall and –

Well, he has to hand it to Douglas. Being the Third of something must have really impressed her, too.

…

Derek doesn't think about it for several days. He doesn't think about anything at all because thinking would cause his mind to flatten and he would have to think about it, so he doesn't even test it.

Katie says, "You're cool with us seeing other people, right?" and Derek nods and says yeah, he's cool, and she's fine with it and so is he, kind of, so it works out. It was easy until it stopped being anything and now everything is fucking fine.

Derek breaches the subject with Douglas after like, four days, which is long enough, he thinks. "How great was Riley's party?" Derek doesn't fucking know if the guy's name was Riley but there's a good chance Douglas doesn't, either.

Douglas shrugs and doesn't even mention that he hooked up with Casey-fucking-MacDonald. Derek doesn't know how to say it without actually saying it and that makes it worse, so Derek just –

"You kind of hooked up with my crazy stepsister at Riley's party."

"Did I?" Douglas doesn't look away from the television. "Sorry, dude. If that's like… weird, or something."

Derek realizes he must sound like a complete douche. "No, it's not – not even, man. Just like – she's kind of neurotic and weird, and like – I don't know, you'd probably not want to… keep going with that."

"Definitely not." He doesn't even skip a beat and Derek gets kind of… angry? "She's kind of like – a fish-kisser. Sorry, if that's weird for you to hear."

Derek suddenly thinks about kissing Casey at all, fish-lips or not, and –

That's definitely how it starts.

…

Rule #8. Casey MacDonald is not allowed to fuck Derek Venturi's friends/roommates/acquaintances. She's not allowed to kiss them, either, and definitely not with tongue, and definitely not where he can see it. Casey MacDonald is not allowed to talk to Douglas [Something] the Third.

…

Funnily enough, Derek sees a lot of Casey after Katie and him sort of fizzle out, and that's the kind of ironic that keeps him up at night.

Dad and Nora basically beg them to come down for the weekend for Edwin's birthday, and yes, you have to come together. They take her car because his car is probably (definitely) not up for the trip, and the entire time they're mostly silent, even devoid of their careless yelling and banter, because all Derek can think about is her fish lips –

That's a weird thought to have, even without saying it out loud.

They see a lot of each other at the house, and then they have to drive back together, and then they end up in the same study hall – it's all very weird, and meticulous, and planned and not planned, and he is kind of going crazy.

He never asks her about Douglas and what kind of Casey-esque conniption fit could possibly have led her to that party, with that guy's tongue down her throat, because he doesn't even want her to know that he knows.

Derek starts wanting to kiss her, just to test the theory, and then he has to sit down and think about what he wants for a long time, and then he draws up a list of negatives, and then he's back to crazy Casey fantasies and that's how he knows he's fucked up.

As if he didn't already know.

…

Casey in a pink bathrobe, untying the knot slowly, tortuously, making him wait –

Casey sprawled out on his childhood bed, almost naked, looking at him like she's going to hit him or say his name, broken in half –

Casey kissing him at a party, so he can test the theory –

Casey in a variety of porno outfits, with a variety of context, with a variety of cheap jazz in the background –

Derek has resolved to waiting around for it to end because it has to, just like it always does.

…

Fate hates him, because Katie calls him pretty late in the night, asking him to "pretty please" come over because Casey has locked herself in the bathroom and Katie really has to head back home in the morning, and she would appreciate it if Derek could come over and babysit her while Katie finds somewhere else to sleep for the night.

Fate has Derek showing up at Casey's dorm at midnight, while Katie gives him this look like don't ask me, and disappears with her bags. Derek sits down in front of the presumed bathroom and sighs.

"You're taking this mentally insane thing too far. Like I knew you should be quarantined, but this –"

"Go away, Derek. I told her not to call you." She sounds like she's been crying and Derek can tell, already, this is what will consume his entire night.

"You wanna talk about it, champ?"

Casey bangs against the bathroom door with her fist, the closest she can get to hitting him. Derek likes this part, but not so much the context. She sniffles and Derek presses his forehead up against the paneling.

"What happened, Case?"

He doesn't expect her to tell him but she says, with a sob, "I failed one of my exams."

And, okay – he gets it. He totally, completely gets it. To Casey, this is astronomical. To Casey, this is – this is like dropping out of school. It's the close equivalent. It's – to Casey, this is the end of the world.

"Which one?" Like it matters, but small-talk always works to calm her down.

"Ugh. Shut up."

Apparently not this time.

So Derek waits outside of the door for like, an hour, barely saying anything to her, and sort of humming to keep himself company. He can hear her breathing on the other side but honestly it's almost as if she isn't there. She stays completely silent while he whistles and hums away, her own private entertainment for the night.

And then she says, "I went to a party when I should've been studying because I was panicking."

Derek remembers Riley's party, and Casey pushed up against the wall, and Douglas, and fish lips –

"It's not like me, I don't know why – why did I even – oh my God." Casey's breath starts coming in short gasps and Derek has to bang on the door, once, to startle her out of hyperventilating. "I just did it because – I thought it would work to be… different. To feel like –"

"Me?" Derek suggests, only half-joking. Casey groans on the other side.

"Why is it so easy for you?"

That kind of scares him because nothing is ever really easy, and that's even more scary to think about, so he just says, "I don't know. I'm perfect."

Casey shifts on the other side. "I kissed a boy I didn't even know and now I hate myself because he probably thinks I'm a prude because –" She pauses as if she's realized talking to Derek about this is totally the worst decision of her life, but then she trudges on, like a champion. "—I didn't let him… do… anything."

Derek sucks in a breath because now he's thinking about – well, by this point he's always – fuck.

"In case you haven't realized it yet – the world is still spinning. So you didn't mess it up too much."

The door unlocks, but Casey doesn't open it yet. She says, "Do you think I'm crazy? Like there's something really wrong with me?"

He says, "We're all a little fucking crazy, right?" which isn't the answer she wants, and isn't a good one either way. "Your brand of crazy is just a special blend."

Still not the right answer, but she still opens the door and looks down at him, half-gratefully, like she's just happy someone was there, and it didn't even have to be him.

She's wearing her pink bathrobe, and all those crazy half-hallucinations feel like they're coming to life right in front of him, except they aren't, because she's not about to let him see what's underneath. Derek stands up and promises himself that if he ever actually did see –

Well, he's still a little fucked up. He blames it on reading Flowers in the Attic in his seventh grade reading class.

Except it's not – well –

Fuck it, it doesn't even matter.

Derek finds something vaguely non-chick-flick-ish to watch from the mounted collection of movies that Katie and Casey have kept, and Casey sits on the complete other side of the couch, her legs crossed resolutely, like she'd do anything not to be close to him.

It's only later, halfway through one of those remakes of a remake, that she inches close enough for Derek to be able to reach out and touch her, which he does, just a little bit, the tips of his fingers wedged between her thigh and the couch, and that's as close as he gets. It's as close as he ever wants to be. He never –

Casey kicks him out, later.

Not before thanking him for being so… chivalrous.

That makes the fantasies feel worse.

…

Rule #9. Derek Venturi cannot, should not, and probably will not ever try to touch Casey MacDonald.

…

Derek sees a lot more of Casey because now they have this weird established role in each other's lives. Like, movie-watching buddies, or confidantes, or – brother and sister, he guesses. And because he's always seeing her, he's not always thinking about her in less-than-compromising positions.

Derek is grateful.

So Casey shows up to his dorm sometimes with popcorn and the newest chick flick that is passable enough as a non-chick flick, and Derek argues with her/yells at her/complains enough with her that she ends up leaving halfway through, or just sitting there in a huff until the movie is over, and it's not so easy, but it's good.

And he eats all the popcorn, which is also pretty good.

Derek takes Casey's calls of desperation, when she really needs someone to help her calm down, because Katie has figured out finally that Derek is miles ahead of her in terms of Casey 101, and she totally takes advantage of it. He has at least six voicemails that all contain some variation of "Casey… crazy… help… please… thanks."

Derek becomes a glorified personal therapist.

The most sickening part, probably, is that Derek doesn't even mind. Because he'd take this over – that. He'd take this over dreams straight out of a trashy porno and at least ten years of psychotherapy for wanting to touch his stepsister in all the wrong places.

So he just… accepts.

…

Derek and Casey are both drunk, which is the first red flag, because, hello, Casey drunk. But, because Derek is drunk, he doesn't care, and thinks this is all pretty fucking funny, because Casey looks so hilarious and loopy and kind of –

No.

Derek just thinks she looks funny.

They got drunk to celebrate the end of the year (which, she reasoned at first, doesn't really count because they do still have exams), and celebrate half a year of surviving college. He muses that Casey didn't survive, as much as she didn't really live, and she hits him twice and tries to say yes I did! and then, of course, it ends up with them drinking wine, because she's too pussy for like almost anything else.

Derek starts to wonder when his life got so goddamn weird.

"Do you think I should dip-dye my hair?" Casey says, thoughtfully, like she's really thought about it.

"What? No. Are you crazy?" The answer is always a resounding yes in his head, but this time it's a genuine question.

Casey frowns into her glass. "Katie had dip-dyed hair and you liked her."

"How many girls do you know have dip-dyed hair?"

"Just one."

"There's a good enough reason, Case."

Casey lays back onto the tile, arms spread out. Derek follows suit and can practically see the floor moving, which is cool, and crazy, and awesome because Casey hasn't even noticed. He's been expecting her to throw a conniption fit basically all night but she mellows out and stares at the ceiling and doesn't say anything crazy.

"I think I made out with your roommate," she says, and then lets out this non-Casey-like giggle, like she has some kind of secret.

Derek can feel something constricting in his stomach and for some reason, he thinks he might puke. "Yeah, princess. Tell me something I don't know."

She props herself up on her elbows and looks at him kind of incredulously, and then pouts. "Oh my God, he told you?"

Derek shrugs. "He said you're a bad kisser."

He doesn't really know why he said that. He just – wanted to –

Derek sort of knows why, but all the answers can only lead to one huge reason that should probably have him locked up or in a straightjacket or on Oprah or something. All the answers sort of lead to –

And he's not thinking about it.

Casey flops back down with a huff and makes some kind of half-groan, half-yell, pulling her hands over her eyes. "Of course he told you. Oh my God. Did he really – no, of course not. I'm a bad kisser?"

She says it like a question and Derek answers, by rote, "God, how would I know?" (Except he really, really wants to know. To… test the theory.)

Casey scoffs at him and Derek feels something else in his stomach and that's how this really happens, if he's honest. He lifts himself up to look at her, and her hands are still covering her eyes, and she's not looking at him at all, and they're drunk enough for it to seem like a mistake, and –

Derek knows that he shouldn't – if only because of the ground rules, of course – and all of those have been broken anyways.

So, he leans down very quickly and kisses her, not so quickly, but just to test it out.

He doesn't notice anything different, and when he pulls back to look at her she's slowly removing her hands from her eyes, and she's looking at him like, well, he's some kind of test she has to pass, and that's scary, so he has to lean down and kiss her again so he can get away from that look, and then she's kissing back, and then everything feels amazing and everything feels okay at the same time and he doesn't know how that's possible.

Derek has never been too religious, except before hockey games and after, sometimes, but he finds himself somewhat believing in a really powerful and awesome God, who's totally looking down on him right now going, "Hell yeah, dude, get it," and Derek is probably too drunk to deserve this but he's not going to pass up the opportunity.

Casey isn't a bad kisser, he finds out, supremely happy that he got to test the theory, and after a few brief closed-mouth kisses he can kind of feel her tongue trying to hang out with his and that's so exciting that he doesn't know what to do about it for like, thirty whole seconds.

It's completely awesome, if he's telling the truth, except Derek is an expert liar.

Casey pushes at his chest to get him off of her and he suddenly realizes that she's his stepsister. Like it didn't even dawn on him until that moment. And he's kind of really fucked up, and he doesn't know what to do, so he just sort of sits there.

She says, "What was that?" like it was all his fault.

And he says, "We're drunk," like that's all it means.

Casey frowns up at him and starts squirming from his grip and that feels so shitty, Derek has to physically recover from the kick it does to his lungs. She hugs her knees to her chest and dutifully refuses to look at him when she says, "I should probably go home."

Derek sighs. "Yeah."

He waits around long enough for her to call a cab and get herself back safely, even if it was just a few minutes of a walk because fuck if he was going to let her do that, and then Derek sort of just… sits there on his kitchen floor, his back against the counter, beating himself up internally for fucking shit up so very well.

…

So days go by, and Casey doesn't show up. Weeks go by and Casey doesn't show up. Derek doesn't go to hers because, well, he knows his place. So it's been a month and they haven't even seen each other at all. Derek is okay with it, except now he's thinking about it more, which makes him not okay with it. It doesn't work out, either way.

He sees Katie a lot, but she stops calling him for Casey-emergencies, which is fine, because they weren't really his favorite part of the week. He doesn't see Casey at all.

Which is also fine.

Derek gets spectacularly wasted whenever he can and doesn't call Casey, or anyone, or even check in with his family, because he doesn't need them to know or care because he doesn't deserve that, not right now.

So they're only linked by a piece of paper, but it's still going to freak everyone out, and God, why didn't they think about that before shoving him and Casey into the same house?

It's science and hormones. Fucking God. That's all it is.

…

Except, like these things sort of go, Derek has a horrible kind of luck with fate, because Casey calls him at six in the morning to announce that Nora is in the hospital about to produce the spawn that is the perfect blend of the MacDonald-Venturi clan. Derek feels kind of sick about it but he still gets kind of excited too, and Casey is already waiting for him out front because they would be dicks if they didn't actually show up.

So, cut scene – Derek and Casey on a road-trip again. It's almost funny but Derek doesn't believe in accidental irony.

He falls asleep on the way there, because she won't even look at him, and her knuckles on the steering wheel are white, and she's glaring straight ahead at the road like it's done something to her, and it all feels so achingly familiar that he might actually throw up.

…

Rule #10. Derek Venturi is not allowed to sleep with Casey MacDonald. Not when they're drunk, not when they're sober, and most definitely not in Casey's car, and most definitely not on the way to see their new baby (half-) brother. Period.

…

"Why?"

The question startles Derek awake from where he's been nodding off in the passenger seat, waiting for her to get back from the gas station with water and snacks.

"Why what?"

Casey still won't really look at him, but now it's obvious she's trying not to, which is kind of cool, because Derek has always wanted to get to her in some way, and now he has. Which is enough to feel like a victory.

"You weren't…" Casey pauses and then enunciates, clearly, "…that drunk. You knew that I was – we were like – you knew."

He kind of wants her to say it, just to admit how totally fucked this is, but she won't, which is still fine. "I was drunk enough for it to seem like a good idea." And of course he'll blame it on the booze because that's what she wants him to do, and secretly he wishes that's all there was to it. Just a drunken mistake.

"That's not how it works," Casey protests.

"How would you know how it works?"

That shuts her up, but she still looks angry, like she totally didn't kiss him back, and that makes him angry, and then he feels like punching something. Because Casey wouldn't even look at him for the first few months of college and now she's acting like –

God, he hates her sometimes. Casey-fucking-MacDonald. He wants to use her name as a swear. Casey.

"Why didn't you talk to me for the first few months?" he asks, tone sort of clipped and trying to keep himself from exploding. Because, okay – exploding at a gas station on the way to meet baby Simon/Michael/Jeremy/Whatzitname is not really on his agenda, but God, he just needs to –

"You didn't talk to me," she points out, and that makes him angrier.

"You didn't even try to –"

"—you didn't even try to –"

"—what are you even talking –"

"Did you even want to see me?"

That catches him because, he doesn't fucking know. That's the worst part. He doesn't know why he's arguing for argument's sake. Like, why does it matter that she didn't talk to him, because he didn't even try, and they both didn't try, and it doesn't even matter. But it does, and Derek wants to do something, wants to hit something, but he doesn't do that, and he just sits there and feels like throwing up and doesn't know why.

He doesn't even answer her. Casey steals one glance and then shakes her head. "You didn't even want to see me. We both agreed on that. You don't even like me."

"Is that something you seriously think?"

Derek wants to do something, anything, so he looks at her, really looks at her, and hates her a little bit, and leans in and kisses her, and it's like the first time, but it's the last time, and it feels all very suffocating, like he'll never know anything else but this. She doesn't kiss him back because that's the way things go.

The words fall from her mouth, "You don't even care."

So, he kisses her again.

And again. And again.

As those things sort of go.

…

Derek doesn't kiss her for the rest of the trip because he knows his place, and they make it safely to Nora and the rest of the family and they all gush violently over baby Simon, and everything is familial and perfect and no one asks, because no one knows, and after two days they make it back to Queen's, and everything is so fucking normal. He doesn't even touch her on the way back (he knows his place). Everything feels like it hasn't changed but he still wants to throw up whenever he sees her, so that's that.

Derek is bad at this, he realizes. College. He should have guessed it from the beginning scene.

…

Casey, in a pink bathrobe, undressing for him, and then right when they get to it, right when she's all pretty and perfect (and, okay – he has to fill in the parts he hasn't seen yet) and naked, she says –

"What the fuck is wrong with you?"

And then it's Casey, naked on his childhood bed, looking up at him, and she looks like she's going to say something low and sweet but she says –

"What the fuck is wrong with you?"

And then Casey at his door, wearing just his t-shirt, looking at him like –

"What the fuck is wrong with you?"

And then Casey, holding baby Simon, looking at him, whispering –

"What the fuck is wrong with you?"

…

It's Casey, at his door, three weeks after Simon, and three weeks after the trip, and she looks like she's going to ask him what the fuck is wrong with you? but she asks him, "Where's Douglas?"

Derek shrugs. "Not here."

Casey steps in without asking and looks at him without asking, which is something she should really do – ask, because he's tired of seeing her, and he's tired of looking at her, because he doesn't even like her and they agreed –

"This is wrong," Casey says, making some kind of finger-gesture between her and him, like she's trying to encompass all of this into one jerk of her hand. "We're not just stepsiblings anymore."

Like it wasn't wrong before, throw Simon into the mix and they're both goners. Except he doesn't even like her. It's just, hormones, and creativity, and curiosity. He's not to blame. It's dad and Nora who did this, even if they didn't mean to.

He doesn't say anything, which spurs her on.

"I don't really know what gave you this idea. But it's not – it's very wrong, Derek."

Derek is very quiet, for the first time, and he knows that he's looking at her in a weird way, and maybe he looks a little crazy but she's been making him go crazy since the day they met. Since the day they became linked by a stupid piece of paper.

"You kissed me back." It sounds pathetic, even to him.

Casey looks at him and, okay – so he's not so pathetic, because she doesn't laugh at him. "I don't know why I did that."

Her chin is raised at him and that makes him want to hit something, so he advances on her quickly, lowering her chin with his thumb just to get her to stop looking at him so fucking defiantly, like she's so much better. Her close proximity makes him feel like he's on fire and he didn't realize it, but he knows now that it always has. Just not in all the right ways.

"Yes, you do," he says, very softly, and she goes all rigid and stiff beneath him, like he's just delivered horribly bad news (maybe he has).

And – cut.

She kisses him first.

…

The sex is good, but that's not the point. It wasn't anything like his fantasies – the good ones, the ones where she's naked and ready and she's not about to tell him he needs psychiatric help, but – that makes it better, he thinks. Because in his non-fantasy, she's still so fucking great.

Afterwards, she wraps the sheet around her and stares straight up at the ceiling, and he tries to look anywhere but at her, and then realizes that's stupid, so he looks at her full-on and doesn't dare move away.

"I didn't talk to you because I was afraid," she admits, still staring up at the popcorn ceiling, not even blinking. "Of… what could happen, when no one was there to make us think about it."

Derek gets it. College meant no family. No family meant he could pretend he and Casey don't even know each other. That would make it easy to –

Well, nothing with her is ever easy.

"I always thought about it."

"Did you?" Casey's voice is skeptical.

"Not… seriously. But I did." Which isn't a lie. What kind of guy would he be if he ignored her?

She's still pretty quiet, and Derek leans in to kiss the corner of her jaw, where it's been set firmly since it was over. It was funny, he thought, how quickly she turned into liquid when he was inside of her, and then now she's back to – this.

"Are you still afraid?" he whispers, so quietly he could've almost not said it. He feels like a child again.

He knows the answer before she looks at him and nods, slowly, and that makes it more of a relief.

Fate has it in for him, as these things sort of go.

…

Rule #11. Derek Venturi is not allowed, under any circumstances, to fall in love with Casey MacDonald. This includes any variation of love. He's not allowed to use the word "love" and "Casey" and "Derek" in the same sentence. Some exclusions can include, "Derek loves to make fun of Casey," or "Casey loves to subtly undermine Derek with her wit."

Rule #12. Derek Venturi hates Casey MacDonald. Period. End scene. One-hundred-percent.