Author's Note: Hey guys! I'm back! Super excited about this, so let's go. A few notes first. This fic is unrelated to Therapy and April's feelings in Therapy do not necessarily represent how she feels in this fic. Separate them in your mind. Second, there will be a lot of "science" in this fic that will, ah-ha, not be real science at all. It mostly surrounds the use of mutagen. I find that the results of mutagen in the show are so wildly unpredictable that not a lot of explanation is truly necessary. Still, I thought I'd preface this by saying that there is no real rhyme or reason to it. Likewise, I've taken a few liberties with Karai's mutation that are not overly important and shouldn't be too bothersome.

Last, I want you all to know that I'm aware that this sort of plot point has been done before. However, I can assure you it's heading in a very interesting direction! Please stick with me. I'm so happy to be writing this story and I want to share it with you all like nothing else in the world!

This takes place directly after Vengeance is Mine.

I do not own TMNT, of course.

Penance.

It was something Donatello wasn't remarkably familiar with in the early years of his life. Unlike Raphael, he largely lacked the temperament or rash thinking that led to poor decisions. Everything was evaluated, tested, calculated and hypothesized. Making mistakes was something he didn't have time for and, to be honest, he didn't have the patience for it either. One of the burdens of being so self-aware, he supposed in retrospect. The purple-banded turtle was constantly re-evaluating himself and his surroundings, searching for faults to be repaired, improvements to be made and new ideas to be formed. He thrived on it, and the knowledge that he had the capacity to better the lives of his family with his work made it all the more satisfying.

So when the day came that Donatello was forced to look back and accept all that he had done, it was with an unfamiliar, gut-wrenching sort of apprehension. One that let him know that penance was more than suffering and regret. It was action.

All of this started because he wanted to fix something. And yet after it was all done, he had no choice but to start from scratch.

He'd be damned if he let that stop him, though.

It was raining again.

Thunder rolled overhead and Donatello looked up, squinting behind his mask as a raindrop landed in his eye. He sputtered and rubbed at it with his knuckles, all the while trying to keep the other open for any hint of movement on the rooftops. She'd been here last, when Raphael and Casey had spotted her a week ago. But she'd been gone in a flash of silver and purple, and now they were on the hunt once more.

Next to him, Mikey shivered. "Dude," the youngest turtle whispered to him. "I am freezing my tail off out here, bro!"

Donnie glanced at his brother and rolled his eyes. "We all are, Mikey, but we still have a whole night to go. It's only been an hour!"

He watched as Mikey scrunched his face up in dismay. "It feels like a million," he muttered, wringing out the tails of his orange mask. Donnie patted his brother's head sympathetically and pulled him up closer to his side. The two of them sat high above their brothers on a tall water tower. It gave them the best vantage point from this part of town, but Donatello couldn't help but wonder if Leo had ordered them to watch out here because it was the rooftop he and Karai had met on all those months ago.

He'd never said as much, but Donnie was observant enough to notice his oldest brother's wistful glances when they'd first arrived. Now Leonardo's sharp gaze followed the line of buildings in front of them, unblinking and unwavering even in the onslaught of rain. Raphael stood nearby, his eyes equally acute. It was nice of Raphael, Donnie thought. He was giving just as much effort to this search as Leo, and Donnie could only figure it was because he hated see his older brother so forlorn.

Donnie's t-phone buzzed and he cringed.

He could feel Leonardo's glare, but he quickly pulled out the device and tucked under an awning to scan the text. He breathed out in relief. "Leo!" He dragged Mikey down from the water tower with him and landed at his brother's feet. "April says she and Casey spotted Karai, come on!" The four of them burst into action with Leo at the front, charging them over rooftops and around fire escapes. Donnie's feet pounded the surface of the roofs, slowing only when he caught wind of the noise from the streets.

"Oh, no..." Donnie skidded to a stop and pointed. "Guys, over there!"

By the time they dropped down to the streets, it didn't matter if they were spotted by humans or not. The damage caused by the Foot Bots and the mutated Karai caused enough alarm on their own. "Get to her before they do!" Leo called out before leaping into the fray. Donnie groaned quietly – this was pure chaos – but he snapped his bo staff into his hand and did as he was told.

The mutated Karai was alarmingly vicious. Foot Bots swarmed around her, grappling with ropes to try and subdue her, but the multiple snake heads that served as her hands snapped and bit at them with every single turn. Donnie ducked just as her massive tail caught a parked car and sent it crashing into a bakery overhead. "I hope those people have insurance," he muttered, barely containing a shriek as a Foot Bot's head flew by and smashed into a mailbox. The sound was deafening on the streets, what with people screaming and car alarms sounding over and over and over again until something (or someone) knocked into it with enough force to shut it up.

"Whoa!" Donnie smashed his bo staff into a Foot Bot's back and knocked it out of the way, but every time he tried to pull one of them away from Karai, he found it was difficult to keep their attention. In fact, it was the first and only time he could remember being annoyed at how little they were fighting. They were too preoccupied with trying to contain Karai, who hissed and shrieked at anyone and everything nearby.

"Ahh!" Mikey went flying through the window of a radio shop and Donnie winced. The rain was coming down twice as hard now and everyone was suffering for it. His head jerked up when he realized Leo was shouting again, but he wasn't talking to any of the turtles. He was trying to talk to her. Again. God, why did he do this to himself?

Donnie jumped up and swatted away a Foot Bot. "She can't communicate with you, Leo!" he shouted, kicking another robotic ninja in the chest. Meanwhile, Raphael was dragging Mikey out of a pile of rubble and supporting him with one arm.

"Mikey's down! We gotta go, Leo!" he shouted over the noise, but Leonardo wasn't having any of it. Donnie sighed deep in his chest and watched as Leo killed one, two, three more bots, slicing them into popping pieces of wires and boxes. The mutated Karai turned on him suddenly and hissed, fangs bared and some sort of a mutagenic poison dotted the ground under her gaping jaw.

"Leo, move!" Donnie shouted, leaping from his spot and knocking his brother out of the way. The two of them crashed to the ground just as Karai's serpentine head jerked up and spotted some fleeing humans. With a great heave, she spat the corrosive poison their way and then sped off just as lightning sliced through the sky.

Donnie shifted from his spot on the ground just in time to see the mutagenic substance fly in the face of the humans. The screams from the woman were so loud and pained, it made him wince. He jumped up to go help them, but his feet stumbled and he could only watch in horror as both of the humans writhed, flailed and clawed wildly at their faces. Then, without warning, their screams shifted into something deeper and more guttural. Their bodies began to change shape and contort, leaving Donnie with nothing to do but watch helplessly as they mutated in the middle of the street. Steam wafted up from the asphalt as the rain continued to pour down, but it did nothing to mask the image of two grotesque mutated figures staggering away, clawing and biting at themselves before they darted into the alleyways.

"Her venom – God, did you see it, Leo?" Donnie whirled on his brother, his mouth parted and his chest heaving. Every inch of him was freezing and now his insides twisted with fear. "Leo, are you even paying attention -"

But when Donnie looked at his brother, Leonardo wasn't watching the people. He was looking in the direction Karai had sped off to, his hands tight on his katana and his jaw clenched. When he finally looked back at Donatello, he could only shake his head. "Let's go."

Donnie frowned. "What about the Foot Bots? I mean, they're... not even attacking us anymore."

"They didn't want us," Leo growled quietly, his chest expanding briefly with a deep breath. "They wanted Karai. Shredder's trying to get to her before we do so he can change her back and make her his again." Donnie's frown deepened. "Surely she wouldn't..."

"How long do you think that retro-mutagen will take?"

The question lingered just long enough for Donnie's shoulders to slump. He slipped his bo staff back into place and shook his head. "I … I don't know, Leo."

Donnie watched as his brother exhaled, and he wondered when he'd last seen him look so tired. It made his chest hurt to see his normally confident brother look so helpless and lost, so burdened. Leonardo shook his head again, and this time he gave no orders. Instead, he simply sheathed his weapons and walked past Donnie without another word.

The rain poured all night.

"Hey there," came a distant voice from the doorway of his lab. Donnie continued to squint through his microscope and muttered something in reply, only vaguely registering that someone had spoken at all. A figure shifted in the desk chair right next to him and a familiar laugh rang through the lab.

"Wow, Donnie. I think that's the most unenthusiastic response I've ever gotten from you," April O'Neil said wryly. A gentle touch to his shoulder had Donnie jerking away from his microscope and it took a moment of dazed blinking before he could smile sheepishly. The redhead seated next to him was a welcome sight on any day. Particularly when she wasn't being trailed by Casey.

"Sorry," he managed a lopsided grin. April tilted her head at him and her smile slowly fell away.

"Are you okay?" she asked, her brow furrowed. She glanced at the busy desk, full to bursting with print-outs, lab samples, slides and containers. She turned back at Donnie and tilted her head, noting the exhaustion that colored his features. "How long have you been in here?"

Donnie hummed as he fought to remember. A glance at the clock told him that he'd forgotten to change the batteries in it. Huh. That was extraordinarily useless, he thought bitterly. "I really don't know," he admitted with a shrug. He quickly turned back to his microscope, mostly in an effort to avoid April's worried frown. Normally, he loved to know she was concerned for him, but this was one time he couldn't afford to be distracted. "I've been re-working my retro-mutagen formula and it's taking me forever," he dragged a three-fingered hand down his face and groaned. "I mean, the sooner I can come up with a dependable formula, the sooner we can get Karai back to normal. April, this is getting serious." He turned to face her, watching as her eyes became illuminated with interest.

"When we spotted her the other night, she was mutating people with her venom. Just out in the streets!" Donnie waved an arm, his voice reaching embarrassingly high octaves in his agitated state. "I don't know how cognizant she is of what she's doing, but she is wreaking full on havoc in the city and I don't know what to do about it!"

"Donnie," April touched his arm. "This isn't your burden alone, ya know."

"Of course it is," Donnie groaned, taking no solace in her touch. His mind was a flurry of images, diagrams, numbers and guessing games. He sighed heavily and pushed away from his desk, rolling his eyes as a cup of pens fell to the floor and scattered. He didn't pick them up. Screw it. "I mean, it's up to me to figure out how to change her back! And then – I mean, it's not the same as when I un-mutated your dad. Whatever that stupid Baxter Stockman did to the mutagen he used on her, it's changed how the retro-mutagen reacts. The few samples I have to work with are giving me no conclusive results, my mutagen supply is limited, and I'm working with equipment that isn't up to the task for this sort of research!" He flailed his arms out, exasperated.

"Donnie!" The redhead jumped up and slowed him again, this time by holding his arm firmly. He looked away from her, shame burning his cheeks. The last person he wanted to list his failings to was April O'Neil, but still, she was the easiest person to talk to. It was one of the reasons he carried such a special place for her in his heart. Sure, maybe she still had this... thing... with Casey, but she had never, ever failed Donatello as a friend. And even now, he felt a curious sort of relief as her hand touched his face in a way he'd never seen her do with his brothers. He let his eyes close for just a moment of respite.

"Look," she said softly, pulling him close. "I know this is stressful and you want to help, but you're going to drive yourself crazy trying to carry all of this on your shoulders. Let your brothers help, Don. Let me help. You don't have to do this all alone."

Donnie sighed softly and let his head fall closer to hers. As so often happened when it was just the two of them, April let him lean against her and place his cheek against hers. It was normally all he needed to calm his nerves, but tonight, his body and mind were set in their stubborn ways.

"You didn't see Leo's face tonight, April..." he murmured, his much larger hands finding her upper arms. April curled her fingers against his cheek. "He's ... I don't know how to describe it. I've never seen him like this before. All he wants is to get Karai back and un-mutated, and I'm the only chance he has. And for Newton's sake, Master Splinter... " Donnie lifted his head and bit his lip, his eyes downcast. April's sympathy was almost too much to bear at the moment.

"He had his only daughter, his only blood, back for less than one night. And now she's gone. How can I let myself enjoy anything when my family's living under the weight of that kind of misery?"

April's eyes crinkled as the sadness lingered between them. She shook her head, as if she knew there was no right answer. Instead, she simply slipped her arms around his shoulders and stood on her tiptoes to hug him comfortably. Donnie touched the small of her back, his eyes open and looking over the top of her head at the doors to his lab.

Even hugging April now wracked him with guilt. Whether he had the liberty to explain his feelings to her or not, at least the girl he loved was safe and at his side. Somewhere, in the distant corners of the lair, his brother and Sensei sat in their respective places and meditated through their grief.

Each knowing that it would never be enough.

"So what's this stuff?"

"It's acid, Mikey. Don't touch it."

"Can I borrow it?"

"No."

"Can I use it to melt something?"

"No... Mikey..."

A dangerously long pause. "What about this stuff -"

"What part of Get Out Of My Lab do you not understand?" Donnie shrieked, wheeling around in his chair.

Mikey froze mid-motion, his hand extended towards a bottle labeled as Highly Volatile. "I just wanted to help!" Donnie growled and hopped out of his hair, eliciting a screech from his youngest brother, who fought to get away.

"You don't want to help! You want to blow stuff up!"

"So? That's all you do!"

Donnie's eyes flashed, his fingers curled into tight fists at his sides. He was working on three days without sleep, an Overdose-worthy amount of coffee and regular meals of pop-tarts. Having Mikey bouncing around his lab like an errant puppy was simply not something he could contend with today. However, at Mikey's jab, he felt his rage threaten to boil over. "That's … all I do? Blow stuff up? You think that's all I do in here?" Donnie shouted. The door to his lab slid open roughly and Raphael entered, munching on one of Donnie's pop-tarts. The purple-banded turtle cringed, trying to control the full force of his frustration.

"What's all the yellin' about?" he asked mildly, eyeing Donnie's overflowing desk.

"I was just trying to help Donnie, that's all," Mikey said plaintively, blinking his blue eyes. Raphael snickered and tossed his wrapper aside. It landed on Donnie's microscope slides. His eye twitching was in such force it was nearly blinding.

"GET OUT OF MY LAB! BOTH OF YOU!"

"Hey, you calm down before I knock the shell out of you," Raphael snapped. Donnie growled, though he did take a step back at Raphael's advance. He couldn't have that numbskull breaking anymore of his things. He needed his limited supplies.

"Get out or I'm telling Master Splinter," he threatened, but as soon as the words were out, he knew they were a mistake.

Raph and Mikey both burst into laughter. "Oooh, I'm tellin' Master Splinter on you!" Raphael mocked, grinning and prodding Mikey. "Geez, what a baby."

Donnie scowled. "I'm trying to do important work in here -"

"Everything you do is 'important' dude," Mikey said distractedly, prodding a beaker until Donnie slapped his hands away.

"Mikey, I will not tell you again -"

"Except you will," Raphael appeared in front of him and folded his arms over his plastron. "Because you're not the boss of anyone here and no one's gonna listen. Lay off'a Mikey." He snatched up the youngest brother by the shoulder and pushed him in the direction of the door.

Donnie stared incredulously. "You beat up Mikey like – every single day!" he shouted at the retreating backs of his brothers.

Raphael scoffed and waved an arm dismissively as they walked out. "Come on, bro," he said to Mikey, just loud enough for Donnie to hear. "Let's leave Donnie to his science fair projects." The door slid shut with a slam.

Their disappearance left Donnie deflated. His anger simmered and settled under his skin, leaving him with a growing headache and a torrent of volatile emotions. It was just like Raphael to brush off his work, to act like he was in here concocting love potions or miracle cures. Like his work didn't matter. His inventions had saved that stupid brother of his so many times! Donnie dropped his heavy arms onto the desk and clenched his eyes closed.

He would show them. He'd cure Karai and all the poor victimized humans in the city, too. And one day – maybe in an alternate universe – they'd appreciate him. If only for a moment.

"No more sightings, then?" Donnie shifted his t-phone to his shoulder and moved around the breakfast food on his plate. His brothers ate in various states of involvement around him, occasionally making the same unruly racket they always managed, so he moved away from the bar and into the living area.

"Not of Karai, no." April sighed into the phone. "Though there have been more people mutated and I think it's definitely her doing. This entire city is going to be overrun if we don't stop her." She paused and then hurried on. "But don't worry, we'll figure it out." Donnie smiled just a bit. She knew him so well.

"Yeah, well, that's a nice thought. How was school?"

April made a sputtering noise that brought a real smile to his face. He dropped onto the couch and winced, shifting briefly to pull a fish hook out of the cushions. What in the world? He shook his head and dropped it onto the coffee table. "I'm finally getting in some study time, but right now I'm working on a term paper and it's just killing me. Normally, I love writing, but the research takes forever to go through and cite. It's so nitpicky!"

Donnie snickered softly. "Yeah, well, I can understand that. I think I have twenty pages of DNA code to sort through tonight."

"Hey, try to get some sleep at some point, okay?" April's voice lilted and Donnie could easily imagine her concerned smile. "You're gonna look like an old turtle man if you keep pulling those all-nighter's." Donnie peered at his green toes from where he'd propped them up against the table.

"I'll live," he said simply, earning a frustrated grunt from the girl on the phone.

"Donnie, don't make me do something drastic." He paused, considering what "drastic" could mean and what he thought might be enjoyable within that range of options. Most of them, he realized with a nod.

"By all means," he smirked.

April laughed on her end of the phone. "Okay, you know what? You're delirious. I gotta go hit the library for a while, okay? I'll talk to you later."

"Be safe," was his customary good-bye, heartfelt and sincere.

"I will," she replied as she often did, equally genuine. The phone disconnected and Donnie dropped his hand into his lap, his head falling back with a thunk. He only had a few minutes to himself before Leo was calling them together for training.

Donnie had always considered group training to be an exercise in numbers. The number of strikes it took to take down each of his brothers respectively, the repetition of the kata, the percentages of success in rolls, jabs and blocks. Master Splinter was always telling him to think less and fight more, but that was easier said than done. Besides, the sort of rhetoric involved in Donnie's fighting was soothing to him in the same ways meditation was to Master Splinter.

Sensei had told him to live in a place in between his thoughts, but those distances often varied. Right now, for example, he felt the walls of his thoughts closing in on him as he fought to stand his ground against his brothers. Raph was being particularly harsh today, perhaps because of their earlier argument, but even his sort of aggressive tactics were no match for Leo.

Cold was the best word to describe him. Donnie and Mikey watched from the side of the rugs as the two brothers dueled, each meeting the other in increasingly heated strikes. Leo's blue eyes narrowed dangerously on Raphael and, in one swift movement, he toppled him over and cut through the air with his blade. It dropped just above Raphael's wide eyes and then stopped, scarcely an inch away.

"That is enough," Master Splinter said from the forefront of the mats. Leonardo pulled away, his gaze sliding to the wall and settling there. Donnie's lips curled in the now familiar frown as he watched him struggle to face Raphael with something other than apathy.

"Good moves," he told him without any real enthusiasm, extending a hand to help him up.

Raphael took it cautiously and righted himself. "Yeah," he said, eyeing his brother. He glanced at the other two, who shrugged in response. As the others moved to disperse, Donnie stayed in his kneeled position for a long moment and studied his family members. Master Splinter was difficult to read, as he had always been, but there was a distinctive slump to his shoulders that Donatello couldn't miss. Leonardo stayed away from the others, and although he had always been the most serious in training, he now carried a stand-offish aura that hadn't been there before. Raphael, normally unconcerned with the moods of others, glanced between the family members with a sort of rare disbelief, his expression put-out and confused, perhaps even sad.

Even Mikey wasn't unaware of the tension that had pervaded the lair over the past few weeks. He tried to joke and prod but was increasingly met with less and less reception. It made the usually boisterous young turtle uncertain and wary. He still laughed often, but it was usually colored by a hint of force, as if he was only laughing because he was used to it. Donnie rose from his feet and drank all of this in. Every ounce of concern, regret, fear and sadness washed over him in a mixture of guilt. His family was on the brink of falling apart.

He had to do something.

Donatello chewed on his lip. He eyed the vial and then turned away, ignoring it one more. His eyes drifted back. He blinked. Calculated in his head, considered the possibilities -

No.

He wouldn't do it. It was morally questionable and the sake of science could only excuse so many things. Donnie turned back to his original formula for the retro-mutagen and began to scour his notes again. There had to be something he was missing, something painfully obvious. He needed to get through – No, that wouldn't work either.

He looked to the vial of red liquid. Should he?

How questionable was it, really? It wasn't as if he was hurting anyone. He didn't steal it. Maybe... No, Master Splinter would never approve. Or would he? It was Karai they were taking about, after all. Maybe it wouldn't be that odd.

Then again, given Master Splinter's aversion to modern day technology, perhaps he would find the idea disturbing and reprehensible. Donnie growled and dropped a heavy fist to his desk, rattling the microscope. He glanced around his lab, as if its montage of medical charts and randomly acquired instruments might help him.

They didn't, of course.

He shuffled his papers, mostly to buy himself some thinking time. He straightened his desk, organized his pens, and damn it all – he kept looking at that little vial. Maybe it wouldn't even make a difference. Maybe he could put it with the formula and nothing could change. Then he'd be worried for nothing.

Or it could change everything.

Donnie scowled to himself. He was wasting time! Images flashed in his mind of the humans he'd seen helplessly mutate right in front of him, of Karai falling limply out of that vat of mutagen and into Master Splinter' arms. Of the wild fear he'd felt when April had been dropped into the mutagen, even though she'd come out unharmed. If she'd emerged looking like Karai...

Donnie shuddered.

This time, when his eyes drifted to the vial, he reached over and picked it up. He turned it in his fingers, his eyes moving over the label and then to his set of beakers. His latest brew of retro-mutagen hissed over the top of a small flame. It was the last bit he'd had, not enough to help Karai but only enough to let him know if he had the right combination of chemicals. If he could create a viable sample, he could hold onto it until he obtained more mutagen and recreate it in a larger quantity.

With a soft growl of determination, Donatello stood and moved over to the beaker. He raised the temperature, calculated the effects and then, with a great heave, he overturned the red vial into the bubbling liquid. He emptied every last drop and then tilted his head, his eyes dancing over the reaction as it sizzled and hissed.

The liquid gurgled and Donnie moved to turn down the heat, the small vial still in hand, but no sooner had he reached for the knob than something happened. Something he felt like he saw in slow motion, even though it was practically instantaneous.

The beaker of retro-mutagen erupted.

When he fought to remember the fateful evening sometime later, he could only recall being blown off his feet and into a wall, a shower of glass and heat raining down on him from above. His head slammed into the opposite wall of his lab, a table toppled near him and nearly crushed his legs, and his last fleeting thought was that damn it, maybe Mikey was right.

Maybe blowing things up was all he really did.

Oh, Newton on a Cracker. His head hurt.

Donnie struggled to push himself off the wall, but he fell back against it with a pained thud. Somewhere in the distance, he heard a pounding on the lab door.

"Uh, bro? You okay in there?" It was Mikey. Donnie winced and fought to coerce his vision into something of use. Everything in his lab was doubled, or at least, everything that wasn't broken into a million pieces. Eugh.

"I heard something kinda explod-ey earlier and I thought I'd come check on ya," the youngest turtle called again through the door. "Actually, I heard it like thirty minutes ago, but I kinda got distracted by this cool show on tv. But I'm here now!"

He continued to jabber through the door, but his words faded away.

Donnie shifted and grunted in pain, wondering why in the world he felt so weak and disoriented. His vision finally settled into place, Donnie turned and braced himself against the wall with a bruised shoulder. However, when he attempted to stand, his knees buckled and he fell once more.

It was then that he managed to look ahead, his eyes clear and his head swimming, only to find himself in the reflective surface of a tool cabinet. What he saw made his face contort in the most elaborate, undignified expressions of fear ever to be seen.

He was looking at a human.

In the kitchen of the lair, Raphael glanced up at the sound of an comically high-pitched scream. He marched into the living room, cereal box in hand, only to see Mikey standing awkwardly at the laboratory door.

"The hell was that?"

"I think Donnie has one of those screaming goats in his lab," Mikey said matter-of-factly. "Which totally bites, because if I don't get to play with it," he turned to the door and shouted loudly. "I'm gonna be real mad!"