"Hey, Dean?"

"What."

"Have you noticed Cas coming and going a whole lot lately?"

"No."

Dean buries his face in a magazine while a pensive Sam sits in front of his laptop. He lightly taps his fingers on a few keys and continues, "I mean, I know he's busy and all, but he seems to come and go more than usual."

"Right," Dean absentmindedly answers.

"I'm kinda worried about him."

"Uh huh."

"Dean?"

"Riight…"

"DEAN!"

"What?!"

"Are you even listening to me?!"

"Yeah! Something about…something."

Sam rolls his eyes with a heavy sigh. "No, come on, this is important. Aren't you worried about Cas?"

"I guess," Dean huffs and turns on his side away from Sam. His body language says he doesn't care, but inside he is worried. He has been worried, but Sam doesn't know why. Sam can't know why. "But he'll be good. I mean, the guy's an angel."

"I'm still concerned. He looks a little off, too."

"Right…"

Dean's eyes close and he frowns to himself. How will he keep this a secret?

"Dean? Do you know something you aren't telling me?"

"What? No! Why would you say something like that?"

"Ugh. Now you're being weird," Sam sighs, putting his face in his hands. "Why am I so out of the loop?"

"Just don't worry about it, Sammy. Leave all the worrying to me, okay?"

Reluctantly, Sam groans and mutters "okay."

And Dean takes on all of the worrying. He worries about Sam and Cas, but mostly Cas because the problems the angel is having right then are way more than Dean can handle.

As Sam says, Cas has begun to frequently disappear from the Winchester's company. Each day turns into a series of "Hello, Dean" and "I must go, Dean." It is unusual, even for Cas. And the little picky nervousness on the angel's face has made Dean grow suspicious. Of course his first concern was that Cas was involved in some sinister plot or under the control of an enemy. Cautious and guarded, Dean took it upon himself to call Cas out on his weirdness one day while Sam was out.

"Listen, Cas, I don't know what the heck is going on here but I don't like it," Dean says loudly into the empty motel room. Cheap floorboards creak beneath his feet and he looks around. "A-and I just wanna talk to you about it, so we can figure it out."

Fwoosh.

"Hello, Dean."

Dean turns around quickly and sees the trench coat-clad angel standing there motionless.

"What's up with you, man?"

Cas furrows his brow and blinks slowly. "There is nothing up with me, Dean."

"The hell there isn't," Dean insists. He gets right up in Cas' face and points at him. "You're being really, really weird. And that's saying a lot considering how weird you always are."

"How am I being weird?"

"You show up then suddenly have to go. You get like…I dunno, nervous? You're different. I don't like it and I wanna know who or what is making you act that way."

Cas takes a step back from Dean. "There is nothing," he says, trying to remain calm. Dean isn't buying it. "There is nothing…"

Dean gets back up into the angel's face and stares at him, mouth open. He notices that there is some color in Cas' face that wasn't ever there before.

"Are you sick or something?" Dean asks in a quiet, gruff voice.

"Angels don't get sick," Cas explains quickly. He stares back at Dean and narrows his eyes. "There is nothing. I have to—"

"Damn it, Cas! You aren't going anywhere!" Dean's shouting manages to keep the angel at bay. "You tell me what's happening to you! Okay?! Damn it, man! I'm fucking worried about you!"

Seeming almost embarrassed, Cas steers his vision away from Dean and sighs. His lips part and he softly states, "heat."

"Heat?" Dean repeats, contorting his face in a confused manner. "You mean like you're on the rag?"

"No, Dean," Cas mumbles. "It is an angel problem. A strange, archaic and widely unknown angel problem."

Now Dean's expression softens and he tips his chin slightly upwards. "Are you uh, gonna be alright?"

"I don't know," Cas says. "It is an elusive disease."

"Disease? Wait, I thought…okay, okay. Back up for a sec; as much as I hate talking about our feelings, let's just, well, talk it through. So what's the problem exactly?"

"I am calling it 'heat'," Cas explains. "For that is the closest human term I have been able to find suitable."

"You aren't bleeding out your ass or anything, are you?"

"No, I am not," Cas answers so straight-faced that, given the situation a little less serious, Dean would have cracked up. Instead he walks to the bed and motions for Cas to sit next to him, which he does. "I have told you before that angels are forbidden from close interactions with humans."

"Yeah, sure. Mortals and orders, God and monsters or whatever."

Cas puts his hands together and looks up. "That is partially the reason, but not the only."

"Ooookay," Dean says, staring at Cas impatiently. "So are you gonna tell me or just keep playing 20 questions 'til I figure it out on my own?

A stiff sigh hisses out of Cas and he closes his eyes with a bit of sensitivity. "Angels have a certain weakness."

"I think they got a couple."

"No, Dean. You don't understand. We have a weakness," Cas looks at Dean for a second and flashes a serious glare in his eyes. "For the entire duration of my existence I had assumed it to be a rumor, but now that I am experiencing it, clearly it is the truth."

"So what is it?"

"Despite our supremacy to man, the necessity of a vessel seems to set us back. And so we have this weakness."

"Right, right," Dean says, getting annoyed now. "The weakness, I know. Are you gonna keep beating around the bush like this?"

"It is a weakness of the flesh."

Then Dean's expression drops. He stares blankly and tries to focus on the angel through narrow eyes. "Come again?" he asks quickly.

"If we build a close relationship with a human, we begin a process that I have called 'heat.' It is a conflict between our vessel and our celestial self, aching for the physical closeness of our human counterpart but driven by forces too grand for mortals to understand at all."

Dean clears his throat. "I don't understand," he says.

Then a graveness comes to Cas and he looks right at Dean sternly and explains, "We begin to require physical intimacy."

"Yeah I get that," Dean says. "But who are you close to that's making you do the nasty?"

"You, Dean."

"WHAT?!" Suddenly Dean springs up from the bed and turns his back to Cas. His face beams bright red, caught off guard by the surprise thrown at him, and he shoves his hands into his pockets. "M-me?!"

"Yes," Cas says, still sitting there, staring down once more.

"You mean, you mean that you've been running from us like that 'cause you're too horny?!"

"I suppose so, yes."

Dean's body begins to quiver. Of course he wants Cas. He has wanted him since day one, he couldn't deny it no matter how strange or "un-Dean" it was, but to actually have Cas respond in a similar manner is a little too mind blowing for him.

"S-so does that mean we're gonna…do it?"

"No," Cas answers shortly. He stands. "This is why I have been avoiding you. When I am around you for too long, my heat flares up and I must have you."

Blushing hard and closing his eyes, Dean mumbles, "Why don't we just have sex?"

"I'm not sure what it would do."

"What d'you mean? This ever happen to any other angel?"

"Yes, once. It is how he learned of it. Chamuel had come to this planet and fallen for a man, which then lead to a great desire for intimacy."

"Okay…and?"

"She fed it. They enjoyed their lust for each other often, but the heat only grew. It was…it was like throwing wood into a fire. The need increased rapidly and she required more until she…"

"What?"

"Until she died."

"Oh," Dean says and swallows hard. "G-gotcha."

"I must go, Dean."

"Wait, Cas. Don't." Dean looks over his shoulder and gazes at the angel, whose face is nearly as bright as his and is even sweating now. Their eyes lock and clear desire pulls at them both, but Cas looks away.

"No, Dean. This is not good."

And just like that, Cas is gone…