Donnie knows, has always known, that his intelligence would eventually paint a giant, red target on the back of his shell, and he had a hunch, after Karai's transformation, that it would only be a matter of days, not months.

He and his brothers thought they had been breaking up some secret Foot rendezvous, some transfer or exchange of goods. Once the extra Footbots erupted from their hiding places, spilled out from the shadows and poured down from the rooftops, Donnie knew he and his brothers were in trouble.

When the fighting well and truly started, he did his best to put distance between himself and the others. They were after Donnie, and given the odds they faced, Donnie wanted to avoid getting his brothers hurt when the Foot inevitably got what they came for.

They're going to take me, Donnie thinks, not for the first time. But those previous thoughts had all been academic. Simple thought exercises to prepare him for the real deal. The reality is finally sinking in now, as the Foot bots encircle him, back him against a wall, and his brothers are so far away from him by his own design.

He is afraid, and fear freezes the blood in his veins as his attacks lose their precision in favor of frantic jabs and blocks. They're going to take me. He wishes he had told someone, mentioned to April or Leo any of his suspicions, about the bounty he could almost physically feel weighing heavy around his neck. But April probably would have tried to assure him that his brothers would never let that happen; and Leo had enough to worry about, trying to lead a team of kids playing at warriors and helping Master Splinter cope with the loss of his daughter experienced three times over.

His staff is suddenly wrenched from his grip, the end of it snapping out to catch him in the throat. He gags and his eyes water. A kick to the center of his plastron sends him sprawling to the asphalt. Mikey might have screamed, Raph and Leo might have called out his name, and Donnie wants to say, please just go, please run, it's not worth it, but the way he's gasping and coughing and choking makes it impossible for the words to come out. In a matter of seconds his wrists are bound and a pinch felt in the side of his neck sends him into darkness.

#####

When he wakes, he finds himself lying on his side, wrists still bound uncomfortably behind his carapace. He tests the ropes and finds no relief there. The throbbing of his head and neck warn him away from moving around too much. He forces his breathing to stay even to give the illusion of unconsciousness, opens his eyes only a sliver to get a better feel for his surroundings.

He's lying in a narrow, dark cell in a dungeon (and of course the Foot Clan would have a dungeon in the middle of New York) lit only by a few flaming torches and slim windows that puncture the stone walls high up near the ceiling. Metal bars blocking his only escape route. He thinks he could easily pick the door if only his hands weren't tied up. His brothers are nowhere to be seen, and he hopes that means they made it home. Two Foot drones stand guard at the dungeon's entrance with their backs to him. They don't seem to notice or care that he's awake.

Slowly he sits up, and the way his body aches tells him that was one of many mistakes today, but he leans his back against the wall and hopes the coolness of the stone will seep away the pain in his head; the bots still don't seem to care. Donnie doesn't bother trying to ask questions, knowing the answers will come eventually, or to demand his own release, knowing that simply letting him walk out isn't going to be an option. Instead, he breathes in through the nose, takes in the stale air that smells of mold and a hint of something metallic, and waits.

#####

His patience turns out to be a double-edged sword when the Shredder himself appears at the dungeon's entrance.

"Turtle," he says, and Donnie commends him for his restraint — the Shredder's voice only sounded a little misted in poison, not dripping with it per the usual. The Foot bots unlock the door for him, allowing the Shredder entrance into the too small cell. Donnie swears for a second that the room gets even colder and narrrower before his rational side catches up. "I have a proposal for you."

Donnie wishes he could shrug a shoulder and say, "Not interested." He wishes he could channel Leo's calm and Raph's bravado and Mikey's optimism that all would be well if he simply believed it hard enough, but all Donnie feels is terrified, feels small and helpless from where he sits on the stone floor. His hands clench into fists so tight that his hands shake, and he's holding his body too rigid and upright, and he can feel even now that his eyes are just a hair too wide, and he knows the Shredder sees it all before Donnie can school his expression back into something like indifference.

The Shredder's eyes narrow slightly behind the faceplate of his helmet, almost in amusement, then he says, "My men tell me that your family has much to thank you for."

Donnie stays silent, but he wills himself not to look away.

"Egg shell smoke bombs. Tracking devices. Battle vehicles salvaged from the cast-offs of a scrap heap. Your ingenuity is impressive, even with third rate parts. And even more impressive is the fact that you have succeeded where those in my employ have failed." The Shredder moves closer, his form cast in shadow. What little light that manages to filter into the room catches on the edges of his armor, razor sharp and deadly. He holds Donnie's gaze for a long moment before the turtle wavers and glances over to the empty space above the Shredder's right shoulder.

When the Shredder suddenly towers over him, Donnie congratulates himself for not flinching.

"You will make retromutagen for me, Donatello," Shredder tells him, and Donnie distantly wonders when it was that the Shredder had learned their names. Did Rahzar nee Bradford, tell him, or was it Karai? "Only then will I release you."

Donnie finally lets himself turn away, staring hard at the wall to his right, but still he says nothing. Shredder's eyes narrow again, this time in annoyance or anger, and for a brief moment Donnie tenses to prepare himself for the Shredder to strike him. Instead, he turns and walks away a few paces. His shadowed form, all sharp edges and cold, glinting metal, reminds Donnie of some kind of childhood nightmare.

"When I was a young man," Shredder says slowly, slicing through the thick silence, "I loved a woman. Someone I once considered a brother betrayed me, and I lost two families, both in the name of revenge. I made decisions, a few of which I will regret until my final breath. But the sum of my choices, of those decisions, has brought me success, has built me an empire. But most importantly, they led me to Karai."

Shredder turns his head slightly, and Donnie stares at his profile. Something in his eyes, his posture, his voice, makes the man look oddly vulnerable underneath that glinting armor. It was strange, to say the least, for the Shredder to show any molecule of compassion, any vague sign of weakness. When Donnie suspected he had been made a target, he tried to ready himself for abuse — physical, mental, whatever. He worked harder in training, preparing his mind and body for the beatings or torture or whatever other scenarios that his overactive imagination supplied. He hadn't been prepared for this. Not for a one-sided conversation with a wounded man.

Then again, Donnie thinks as he tries to shift quietly into a more comfortable position, he's still trapped and bound in a prison cell, so the Shredder was clearly not interested in becoming friends. Somehow that spurs something in him, and some of his fear thaws.

"You are here to correct a mistake." Shredder's tone sharpens again. "You are here to cure my daughter."

Donnie lets his mind work, thinks of all the scenarios and possibilities that this brief alliance with the Foot would wrought. None of the outcomes looked favorable, but what choice did he have? He could tell the Shredder where exactly he could stick his offer, in the spirit of Casey Jones, but that would just end badly. The Foot Clan holds all the best cards. He knows he's at a huge disadvantage, his cleverness being his only bargaining chip, and despite everything, he really does want to cure Karai, for her sake and Splinter's sake. And he needs this truce, needs the Foot's supply of mutagen now that his supply has run out, and he briefly entertains the idea of smuggling cannisters of the substance back to the lair to cure Timmy and Spike and everyone, too.

Slowly, he gets to his feet and knows that Shredder is keeping a wary eye on him. Donnie takes a deep breath to center himself; when he speaks, his voice is hoarse and uneven from the blow he took to the throat, but it's as steady as he can make it. "If I help Karai, you'll let me go home. We're agreed on that? I won't be followed, and you won't harm my family."

A nod in reply. "No action will be instigated against them."

"If I do this, I'll need equipment and plenty of mutagen."

Another nod. "You will be given access to a laboratory."

"I'll also need access to Stockman's notes. I need to know what he did to the mutagen that Karai was exposed to—"

Shredder waves a hand. "Granted."

"— but I refuse to work with him. He's an idiot."

He seems a little amused by that. "Fine, but you will still be supervised."

Donnie hesitates, then asks quietly, "What about Karai? What will you do with her?"

"What concern is that of yours?"

Donnie steels himself and squares his shoulders, meeting the Shredder's mismatched eyes without hesitation. "She's family," he says with every ounce of confidence he can muster. "She already made a decision to stay with us, didn't she? I want her to come home with me. She's our sister, and her real father—"

This time the Shredder does strike him, the back of his metal gauntlet slamming against the side of Donnie's head. He reels, and his plastron smashes against the floor, his teeth snapping together against the inside of his cheek when his chin meets stone. For what feels like ages, he simply lays there, head spinning. Master Splinter had taught them all how to take a hit, but that was something else entirely — and even worse, he knows the Shredder had held back on that blow, and still Donnie was dazed.

"You will make the retromutagen." The Shredder coolly readjusts his gauntlet, as if nothing had happened. His eyes were impassive as Donnie struggled into a sitting position, as he spat blood from his mouth. "You will have the supplies you require. And you will have care to never question where my daughter belongs. Only then will I allow you to run back to your disgusting sewer, to your abomination of a family." He spits the word out like it's the most disgusting thing he's tasted. "Am I understood, turtle?"

Donnie glares at the floor before him, spits more blood before murmuring, "Understood."

Shredder nods in satisfaction and sweeps out of the room. He says, "You begin tomorrow."

The bang of the cell door slamming shut echoes in Donnie's head the rest of the night.

#####

As it turns out, the lab is just Stockman's old roost, cleaned up from the fire that had ravaged it the night Karai was mutated. Already Stockman had made it filthy again, spent candy bar wrappers and overturned waste bins standing out in stark to the blackened workspaces, floors, and walls. The equipment looks new, Donnie notes, and there are a few glass cages that weren't there before along the wall, but the giant vat of fresh mutagen still stands in the center of the warehouse, casting everything in an eerie green-blue glow.

The next few days pass in a blur. Each day, Donnie is led in by a cadre of Foot bots, just as Stockman would be kicked out, screeching and buzzing his outraged protests. Donnie would let himself be entertained by the display for only a brief second before a Foot bot would undo the binds on his wrists. The turtle would only have a few moments to coax life back into his aching shoulders and fingers before his appointed guardian, usually Rahzar, would bark, "Get to work, turtle."

Each day, Donnie would flirt with the idea of attempting an escape. He doesn't think he has Leo's mind for practicality and strategy, Mikey's instinctual skills and speed, or Raph's strength and dogged determination; what Donnie knows he has is an academic intellect to rival pretty much anyone in New York. He knows math, knows statistics, knows how to create. He thinks of making smoke bombs or blinding powders or flash-bangs to aid his escape. But then he would take stock of his enemies, would see the smirk of his appointed guardian just daring him to try to leave; he would take note of the bots watching him carefully on the floor, of the bots lurking in the rafters above him, guarding the high windows; he would listen for the bots patrolling outside, blocking him from getting too far and his brothers from attempting a rescue.

He knows his odds. They aren't good.

#####

On the fourth day, the commotion outside the lab makes Donnie's pulse race, adrenaline flooding his system. They're here, he thinks, and his fingers twitch as he cautiously looks around for something he can use as a makeshift staff. Donnie barely breathes as he listens, willing his brothers to burst in, and his chest aches with how much he's missed them. He wants so badly to be home and for Master Splinter to smile and wrap him in one of his hugs, rare but always warm and assuring, to signal the end to this ordeal.

Instead, the Shredder appears. He cradles Karai in his arms like a sleeping child, her head tucked securely beneath his chin. The snake head that acts as her hand hangs limply at her side. Once he secures her into one of the glass cages, the gentle fondness that might have been in his expression falls away when his eyes meet Donnie's.

"I expect a cure soon," Shredder tells him, and the threat hangs heavy in the air when he finally leaves.

#####

With Karai there, Donnie knows he can't stall anymore. On the fifth day, he informs Rahzar that the retromutagen is ready, and he starts planning his and Karai's escape in earnest. He's not allowed anywhere near the containment unit where she's kept, but she watches him calmly, warily, with none of the murderous intent from the first night she transformed present in her eyes. She hasn't seen her speak a word in the little time she's been there, though Donnie knows she's capable of it. He hopes that she'll remember who she was — Hamato Yoshi's daughter — when she turns back.

#####

It takes seven Foot bots to restrain Karai with capture poles, the cables looped around her neck and wrists and tail to keep her from lashing out at them. Two more Foot bots hold Donnie firmly by the elbows, but this time they forgo the ropes. Small blessings, Donnie thinks, though the thought is quickly erased from his mind when the Shredder enters the burnt-out lab.

Rahzar hands the vial of retromutagen to Shredder as he passes, and for his part, the Shredder handles it with utmost care. He strides up to Karai, who hisses at him and prepares to spit venom before one of the cables around her neck is tightened in warning. The Shredder pretends not to notice.

"This is one full dose?" he asks, not turning to Donnie even as he addresses him. He carefully turns the vial in his hands, examining the orange substance.

"It is. It has to be absorbed into the skin all at once for maximum effectiveness."

"I heard your friend's father was cured with only a splash," Shredder says, almost conversationally. "Wouldn't it be more efficient, in that case, to lower the dosage?"

Donnie shakes his head. "The odds aren't good. It was a one in sixty chance that such a small dose would have worked." One in sixty-two-point-three-three-repeating, his mind supplies helpfully. Important to be accurate.

"What would have happened if the retromutagen had not worked? Side effects?"

Suddenly Donnie is aware of Karai's eyes on him, and it makes him nervous.

"Nothing, he just would've stayed as he was," he says after a hesitation, mouth dry. Something in him makes him stress again, "Karai needs to take it all of it for maximum effectiveness. All at once."

The Shredder nods but says nothing in reply. Instead, he gestures for the Foot bots to force Karai into something like a kneeling position. He uncorks the vial and pours the retromutagen onto Karai's forehead. When half of the vial is spent, he quickly recorks it.

"What are you doing?" Donnie yells, jolts forward only to be restrained by the strong, inhuman hands on his forearms. "You have to give her all of it! It might not work—"

And Karai screams.

#####

The transformation isn't elegant or smooth, and it's not at all easy to watch, but Donnie forces himself to do it. He's terrified that maybe there was something he had overlooked, some secret ingredient Stockman had put into the substance used for Karai's mutation that was making her reject the retromutagen. She writhes and shrinks and stretches on the ground, the restraining poles having been pulled away to avoid hurting her any more than the retromutagen already is. He can hear the bones reforming and the quiet clink and shift of scales falling away and softening into human flesh, all of it laced with her grunts and cries of pain.

When it's all over, white scales are pink skin, and Karai kneels on the floor, breathing heavily. Donnie tries again to move forward to check on her but the grips on his arms are steadfast, and this time the Foot bots force him to his knees. The Shredder trades the vial to a Foot bot for a black robe, which he drapes over Karai's bare back, and he kneels beside her. He places one of his hands on her shoulder, and with the other he removes the faceplate of his mask. He whispers her name, and Donnie marvels at the gentleness there.

Slowly, Karai puts her arms through the sleeves of her robe, tying the sash firmly around her waist. The Shredder helps her to her feet, and she leans heavily against him. Donnie swallows around the lump in his throat when he realizes he doesn't see anything of the Karai that had briefly stayed in their home and had for a few moments slotted awkwardly into their family. She says nothing for a long time, just taking in her surroundings with an indifferent expression. Finally, she asks, "Father, what happened?"

Shredder relaxes entirely. "That can be explained later," he says. He turns to the Foot bot and Rahzar, and suddenly his expression hardens. "You will leave the vial with Stockman when we are finished here."

"What are you doing?" Donnie finds himself asking again, but this time his voice is distant and thin.

When the Shredder turns his attention to him, Donnie feels that familiar chill in his veins. The Shredder smiles, though it looks more like a grimace to Donnie, and the turtle's muscles tense. "It is no concern of yours. You will be released, as per our agreement."

Karai's head tilts in curiosity, a hand pressed against the inside of Shredder's elbow. "You're going to have Stockman reverse engineer it, aren't you?" she asks lightly.

Dread plummets in his stomach like a stone. "Deformulate," Donnie corrects automatically.

Karai ignores him and smirks her signature smirk. "You're going to use it on the rat and the turtles."

The fond expression on Shredder's face when he looks down at Karai makes Donnie feel sick, but it tells him everything. Karai laughs, and it echoes in the warehouse like the chiming of delicate bells. She says, "Are you honestly going to release him, Father? I think he knows too much, now."

Donnie's eyes widen. "Karai?"

"I made an agreement, daughter."

"Perhaps you did, but I did not," she says primly. She holds a hand out to a Foot bot, and on the Shredder's nod, the hilt of a tantō is placed in her palm.

"Karai—" Donnie's voice cracks as she stands over him. He searches her eyes and what he sees there makes him pull against the grip the Foot bots have on his arms. A third comes up behind him and puts its weight against his carapace. It pulls back on the tails of his mask, forces him to expose his neck.

"Prepare yourself," Karai hisses. She raises the knife up and across her body and angles it downward, hacking off a Foot bot's arm with one blow as Donnie surges to his feet.

And suddenly his brothers are there, leaping down from the rafters, and Donnie smiles for the first time in ages.

#####

"Where have you been?" Splinter demands when they appear at the turnstiles. Fear and relief and panic make his voice angry and sharp, and his words echo harshly against the walls and ceiling of the silent room. "I ordered you all to stay here until we perfected the plan. What have you done?"

The lair is dark, the main area lit with only a few candles as Splinter kept his vigil. The boys file in quietly, two breaking off to collapse onto the couch in the main area, and the air of defeat overpowers the faint tinge of blood and sweat.

Miwa is the last to enter, and the coil of panic in his gut loosens slightly while his hands shake. He gently brushes the backs of his fingers against her cheek and she leans into the touch. Time and again they have been in this position, and for just a second he dares to hope for permanence. She is human and whole, and the eyes are her own, not those of the beast that claimed her body. But she will not look at him, and there's an uncharacteristic glossiness in her eyes.

"What has happened?" Splinter asks. He turns back to the boys and sees for the first time their tear-streaked cheeks, the far-off look in their gazes – they're in shock, Splinter's mind supplies – as he steps toward them.

One of the boys finally moves forward, and like Miwa, he refuses to meet Splinter's eyes. He takes a deep breath and whispers, "I… we weren't fast enough." Leonardo's voice is thick, and his entire body is shaking in his effort to maintain control. Splinter can feel the cold vice constrict around his too-fast beating heart and he's finding it hard to breathe.

Miwa murmurs, "He understood that sacrifices must be made in war."

"He's an idiot," Raphael barks, and Splinter knows how often his son uses his anger as a crutch and pays his tone no mind.

"He was trying to protect us," Michelangelo says, and his voice is pained and filled with too much sorrow. He moves to face Splinter, and his eyes are wide with fear and desperation and shine with unshed tears. "You have to understand, Sensei. He just wanted to protect you."

"What has happened?" Splinter asks again, whispered this time, and a cold vice tightens around his heart.

#####

"Do not fool yourself into thinking you've won." He's the last man standing, but the turtles and Karai know the Shredder can defeat all of them easily if they stay too long. The floor is littered with the mechanical innards of dismantled Foot bots, and Rahzar is underneath a pile of rubble somewhere, out cold.

The exit is at their backs, and they're so close to escaping and making their family whole again that Mikey could cry with relief.

"I will not rest until I have you again." His mismatched eyes fall on Donnie and that half-smirk on his lips spurs Leo and Raph to tighten ranks in front of their brothers, weapons drawn and bodies tensed for action. Even Karai twitches the blade in her hand, but all Mikey thinks to do is watch Donnie, whose face pales and eyes widen.

"Next time, I will not extend to you the same kindness as I have these few, pleasant days," he continues. "I will break you, turtle, and you will create more retromutagen for me, and you will sing to me all of your secrets. When that happens, I will make your master watch, one-by-one, as I turn you back into the defenseless, pathetic house pets you truly are. I will force him to bear witness as you all writhe and scream and plead until you're nothing more than vermin the size of my hand, and I will make him grovel and beg for your lives before I crush each of you in my palm. This is my revenge, and Hamato Yoshi will know true pain once I have finished with him."

The Shredder's eye shine dangerously, and his smile makes Mikey feel physically ill. "Your mind is more dangerous to you and yours than you realize, boy. You, Donatello, and you alone, will be the undoing of your miserable family and of the disgusting creature you call a father."

Donnie looks down at the floor, and it takes Mikey a second to realize he's shaking like a leaf and breathing too heavily. He grips the half-full vial of retromutagen tightly to his chest, and Mikey reaches for him as he whispers his name.

Karai cries out a warning as Rahzar leaps out at them with a roar, driving straight through their unguarded flank, straight for Donnie. Leo and Raph are shoved aside like paper dolls and Mikey and Karai are flung through the open doors, and Rahzar's claws are slicing through the air, reaching for the vial.

The boys scramble back to their feet and dive back into the fray.

When the glass shatters, Donnie knows his odds, and he knows they aren't good.

#####

When Leonardo moves and reaches into a makeshift satchel at his side that Splinter is only now noticing, Splinter can almost feel the air in the room thicken. Miwa bows her head, and Splinter hears the way his children's breaths hitch when content of the bag is revealed. Splinter forces himself to look down to see what Leonardo clasps carefully in his hands – he does it quickly, hoping speed will soften the inevitable blow. He can only stare for what feels like ages before his legs buckle, and somehow, Raphael and Miwa manage to steady him in his descent to the floor. Michelangelo weeps all the more and moves away.

A small box turtle gazes up at Splinter, a stained and tattered purple bandana wrapped around its shell, and it – he – it stares at Splinter with red-brown eyes that are both familiar and unfamiliar and too unrecognizing and calm and dull, and nothing in its blank gaze shows even a hint of his once brilliant son's mind.

Splinter is only distantly aware of Leonardo's pain-filled voice when he says, "He's gone."