Chapter Text
Donnie had been working on a new stabilizer formula when he heard something shatter behind him.
His head snapped up.
Mikey stood in the middle of the lab, frozen, his wide eyes darting between Donnie and the glass shards at his feet. A dark green liquid had splattered across his plastron, glistening under the harsh fluorescent lights.
Donnie’s stomach dropped.
“Mikey—don’t move.”
Mikey barely heard him. His breathing had gone shallow, his fingers twitching. His whole body stiffened, then gave an abrupt jerk—like something had just punched him from the inside.
“I—I don’t feel so good, dude.” His voice was thin, strained.
Donnie’s brain was already racing through possible reactions—Was it mutagenic? Neurotoxic? He needed to get Mikey under a scanner now.
But then Mikey gasped, choking on air.
His hands clawed at his chest. His ribs were compressing, his body pulling inward. His limbs jerked violently, his knees buckling as he hit the floor. His fingers spasmed, then shrank, muscles seizing as his bones contracted.
“Holy—” Donnie’s voice broke. No no no—
Mikey convulsed once, then his whole body began to fold in on itself.
Donnie didn’t wait to see the rest. He bolted from the lab.
“LEO! RAPH! GET IN HERE—NOW!”
His voice cracked through the lair like an alarm.
Leo and Raph were already on their feet by the time he skidded into the room.
“What happened?” Leo demanded.
Donnie was panting, trying to form words. “Mikey—he—he mutated!”
For a split second, everything went still.
Then Raph was moving first, shoving past Donnie, his eyes wild. “What?!”
Leo was right behind him, face pale but controlled. “What do you mean mutated? Mutated how?”
“I—I He—!" Donnie stammered, following them back toward the lab. "The vial—it was an unstable compound—I don’t even know if it was meant for exposure—”
They rushed inside.
Raph’s breath hitched. “Where the hell is he?!”
Donnie’s chest clenched. He swallowed, pointed a shaky hand to the floor.
Leo followed his gaze—then froze.
Because right there, in the center of the lab, was a tiny baby turtle.
A soft, orange-masked hatchling, curled up in the middle of their tiled floor. His chubby little hands twitched in his sleep. His shell, once cracked and scarred from years of battle, was smooth and perfect. He let out a tiny, content sigh.
Silence.
“…What,” Raph said blankly.
Donnie felt like he was going to pass out. His brain knew what had happened—he recognized the compound now, understood the forced age regression at a cellular level—but the knowledge didn’t make this any less terrifying.
Leo took a slow step forward, his movements cautious—like approaching an unexploded bomb.
“Is that… Mikey?”
Baby Mikey yawned in response, stretching his tiny arms.
Leo exhaled, pressing a hand over his face. His body sagged slightly, like his brain had short-circuited. “Okay,” he muttered. “Alright. I thought you meant he died.”
Raph, who had been seconds from an aneurysm, let out a strangled breath. “I’m gonna kill him when he turns back.”
Donnie was still standing there, staring. His hands were trembling. His mind was a blur of panic, calculations, and sheer disbelief.
Then baby Mikey rolled onto his back, kicking his little feet. His big blue eyes blinked up at them. He giggled.
And just like that, Donnie’s knees buckled.
***
Mikey was a baby.
Like, an actual baby. Tiny, round, soft—his chubby fingers flexed uselessly at the air as he squirmed on the floor, his shell smooth and barely developed. His bright blue eyes were big and unfocused, blinking up at his very not okay brothers.
Leo took a long, slow inhale. Then turned to Donnie. “Explain. Now.”
Donnie was still frozen in place, his hands hovering in front of him like he didn’t know what to do with them. “I—” He swallowed. “It was a mutagenic compound. Unstable. It must’ve triggered age regression on a cellular level.”
Raph, whose blood pressure had yet to return to normal, pinched the bridge of his nose. “So you’re saying that mutagen just de-aged him?”
“Yes!” Donnie snapped, his voice cracking slightly. “But it’s not just de-aging—it’s forced biological reversal. His body is literally rewinding itself!”
Leo’s jaw tightened. “So how do we fix it?”
Donnie hesitated. “We… wait.”
“Wait?” Raph’s eye twitched.
“I don’t have a counter-agent for this, Raph! It’s not like he’s sick—his cells are locked in a state of regression. We just have to wait until the process stops naturally!”
Leo exhaled, rubbing a hand down his face. “How long?”
“The compound’s half-life suggests… weeks. Maybe a month.”
Silence.
A full month of baby Mikey.
A soft cooing noise broke the tension.
The three turned back to the tiny turtle on the floor.
Baby Mikey had successfully rolled himself onto his stomach, his head bobbing unsteadily as he tried—and failed—to lift himself up. His tiny fingers scrabbled against the cold tiles, his little legs kicking uselessly behind him. His face scrunched in frustration, then relaxed as he gave up and just… sucked on his fingers instead.
Leo’s shoulders dropped. “Oh no.”
Raph scowled. “Nope. No way. I refuse to do this. I am not changing diapers.”
“I don’t think he wears diapers,” Leo muttered, but Donnie cut in before they could spiral.
“We need to move him somewhere safe,” he said. “The lab is not baby-proof.”
Leo nodded, already moving. He carefully knelt down, hesitating for only a second before scooping Mikey up.
Mikey made a surprised noise, tiny hands flailing. Then, after a few seconds, he cooed and buried his face in Leo’s chest, curling up against the warmth.
Leo went still.
Donnie and Raph stared.
Leo swallowed. “I… don’t know what I expected.”
Raph crossed his arms, unimpressed. “Yeah, yeah, he’s adorable. He’s still a pain in my shell.”
Baby Mikey hummed softly, blinking up at them with drowsy eyes. Then, very slowly, he reached for Raph—his little fingers waving clumsily in the air.
Raph twitched. “Don’t you dare.”
Mikey’s chubby fingers brushed against Raph’s arm.
Raph tensed.
Mikey gurgled and grinned at him.
Raph sighed deeply, glaring at the ceiling like it personally offended him. “Fine. Give him here.”
Leo smirked but handed Mikey over. Raph took him carefully, holding him awkwardly at first—then more securely when Mikey immediately tried to snuggle into him.
“…He’s kinda warm,” Raph muttered.
“He’s a baby,” Donnie deadpanned. “His metabolic rate is higher.”
Mikey yawned, his tiny face scrunching up before relaxing again. His fingers curled into the fabric of Raph’s bandana.
Raph groaned. “I hate this.”
Leo huffed a quiet laugh, but the humor didn’t last. His expression grew serious as he turned back to Donnie. “This is temporary, right?”
Donnie hesitated before nodding. “Yes. The mutation isn’t permanent. We just… have to be patient.”
Leo exhaled. “Alright.” He turned back to Raph, who was not giving Mikey back. “Then we figure this out. Together.”
Raph grumbled under his breath, but he didn’t protest.
Mikey, blissfully unaware of their distress, sighed contently in his brother’s arms.
They were in for a long month.
***
Splinter had prepared for many things in life.
Raising four mutant sons in the sewers of New York had thrown plenty of chaos his way, but as he stood in the dojo, looking down at the baby in Raph’s arms, even he had to admit—
He had not prepared for this.
“…Explain,” he said, voice calm but firm.
The three remaining turtles stood at attention like scolded children. Raph, however, was still absently bouncing Mikey in his arms—though he didn’t seem aware he was doing it.
Donnie took a deep breath. “Mikey had an accident in my lab.”
Splinter’s ears flicked. “An accident.”
“Yes. A, uh… mutagenic accident.”
Silence.
Mikey let out a quiet gurgle.
Splinter’s whiskers twitched, but his voice remained steady. “Continue.”
Donnie adjusted his goggles, clearly struggling to keep his voice clinical. “The compound that spilled on him wasn’t normal mutagen—it was an unstable regression formula. It forced his entire body into a state of reverse aging.”
Splinter’s expression didn’t change, but the air in the room shifted. “Is it permanent?”
“No.” Donnie hesitated. “But it is long-term. Based on the compound’s half-life… we’re looking at a month before he turns back.”
A month.
Splinter exhaled slowly through his nose.
Leo took a cautious step forward. “We’ll take care of him, Sensei. We just need to make sure he’s safe until then.”
Splinter studied them for a long moment before stepping forward.
Raph instinctively held Mikey a little closer, but Splinter simply extended his arms. Raph hesitated, then carefully passed the baby over.
Mikey blinked up at him, big blue eyes round and curious. His chubby fingers flexed before gripping onto Splinter’s fur, tiny hands clinging to it like he knew this was someone important.
Splinter ran a careful claw over his shell. Mikey gave a soft sigh and snuggled closer against him.
“…He is unharmed,” Splinter murmured, mostly to himself.
Donnie nodded. “Yeah. He’s completely healthy. Just… small.”
Splinter was quiet for a moment before looking at his sons again.
“You will all take responsibility for this.”
The three straightened. “Yes, Sensei.”
“You will protect him.”
“Yes, Sensei.”
“And you will handle all of his needs.”
“Yes, Sen—”
A sudden squish noise filled the air.
Raph froze.
Leo stiffened. “Oh no.”
Donnie immediately stepped back. “Oh, gross—”
Splinter barely reacted. Instead, he simply turned on his heel and began walking toward his room.
“Sensei…Where are you going?” Raph demanded, panic creeping into his voice.
Splinter paused at the doorway, glancing back with the kind of exhausted look only a parent could have.
“I raised four of you,” he said flatly. “I have done this before.”
Then he disappeared into his quarters, leaving three dumbfounded turtles standing in his wake.