Cryptic a/n: Think Asa Butterfield (Hugo in "Hugo", or, for you cool people, Mordred from Merlin).
The Doctor's entire body convulsed. Amy tried to run forward, but Rory held her back. "What's happening!" Amy sobbed. "Captain, what's happening?"
Jack was watching the Doctor sadly. He shook his head. "This isn't supposed to happen."
"What's not supposed to happen?"
"You said you saw him die," said Jack in a low voice, turning away from the Doctor. "In this body." He turned back to see the Doctor's hands and face being encased in clouds of glowing golden light.
"Captain, what is happening?" Amy said again.
"He's regenerating," said Jack. "When his body is dying, he regenerates into a new one. It's okay. He'll be fine."
The Doctor screamed and collapsed to the floor. "Nooooo!" As the scream continued, it rose in pitch. They could see a face now. It was smaller than his last, and capped by straight, dark brown hair. The Doctor screamed again and curled in on himself, scrunching into a ball.
He stayed there, breathing hard, his arms over his head. He must have been smaller, because his jacket was now baggy and wrinkled.
"Doctor?" said Jack tentatively. He walked slowly forward towards the Doctor's curled up form. He placed a hand on his shoulder. "Doc?"
The arms loosened, and the Doctor raised his head, looking up at Jack.
"Oh," said Jack, leaning back on his haunches. "Wow."
"Doctor?" Amy and Rory also came forward. "Oh, my," whispered Amy. "Doctor, your…"
The Doctor raised himself to his feet. His head reached Jack's chest. He looked no older than twelve. He looked up at Jack. "Give me a mirror," he said softly.
"Doctor, I don't think that's so-"
"Damn it, give me a mirror!" shouted the Doctor, just as angry as they'd ever seen him. Still the same Doctor.
Reluctantly, Jack turned the mirror attached to the console towards the Doctor. He consulted himself. "Blimey. I look like Mordred." He brushed his bangs in front of his eyes, then pushed them back. "I met Mordred once. Nice kid. Until he killed Merlin of course." He frowned, looking up at the ceiling. "Merlin wasn't so nice. He was kind of…pompous. And old. Blimey, he was so old. Right annoying old bugger."
The Doctor whipped around. "What? What's so funny?"
"You're the kid Doctor," said Jack, grinning. We should call you Doogie Howser."
"That isn't funny," the Doctor snapped. His eyebrows furrowed and his mouth turned down. But it was funny. Quite considerably.
I didn't intend for this to go anywhere, but now I'm kind of evilly imagining all the fun I could have with the Doctor trying to persuade adults that he's really the Doctor. And possibly returning to say Martha and/or Rose, just because I'm sadistic.