Disclaimer: I do not own Doctor Who or Harry Potter. I am not that awesome.
"Jack, hurry up!" The Doctor ran for the TARDIS, fumbling with his key.
"Doctor, wait! I don't understand," Jack shouted. "The gun, it didn't do anything."
"The effects won't become apparent yet; they increase exponentially." The Doctor ushered Jack inside and then charged down a corridor. Jack shrugged and followed. After a few minutes, he was thoroughly lost. In all his time travelling with the Doctor, he never explored more than the corridors immediately adjacent to the control room; there had never been any need, everything Jack wanted or needed was right there.
"Here, here, here!" shouted the Doctor and threw himself through a door. Jack followed and found himself in some sort of lab. The Doctor was scrambling, pulling chemicals out of cabinets and hastily measuring them out before dumping them into beakers.
"Doctor, what is going on?"
"That blast, the gun… it's a de-ager. Literally youths you out of existence," the Doctor explained, still mixing chemicals. "It's a favorite of special ops teams and assassins because it doesn't leave a body."
"And you're doing… what?" asked Jack, watching the Doctor stirring the contents of two beakers, one with each hand.
"Stopping the de-aging. Here," he thrust one of the beakers into Jack's hand, "drink this." The Doctor took his beaker and gulped it down, grimacing at the taste. Jack followed suit and was pleasantly surprised by the light flavoring of pears.
"So we're fine now?" he asked. At that moment, there was a loud popping noise and a burst of light. Jack looked around, blinking. Much to his astonishment, the Doctor, standing before him, looked like the original Doctor. That is, the first Doctor Jack had met. "Uh, Doctor, I don't think it worked. You're all ears and nose again."
"Of course I am," he replied in his old, thick Northern accent. "No, the mixture we just drank worked perfectly." He started mixing more chemicals together. He paused for a moment in thought, then reached into his pocket and pulled out his glasses and started working again. "It wasn't to get us to stop de-aging, I'm starting that one now. No, that was to preserve our personalities."
The Doctor turned to Jack, an eyebrow raised. Jack had to stop himself from laughing out; it was way too surreal seeing his original Doctor still wearing a pinstripe suit (that was too small for him now) and thick plastic glasses. "The mixture I'm making now will take at least an hour to make. By that time, you'll probably have de-aged till before you met Rose and me, you'll possibly even have de-aged until the point you were still working for the Time Agency. You'll still know me, the gun doesn't remove memories, but from what you've told me, and what I know of the Time Agency, it won't matter; your personality would be so different, it would seem like we're strangers.
"I, on the other hand, will continue to de-age and swap bodies, each time for the previous regeneration. I always loved who I was at the time, but with hindsight, I know that some of them were difficult to get a long with, and they all had different preferences for preparing chemicals. I'd rather not be interrupted and have to start basically from scratch every time that happens."
Jack nodded his understanding and the Doctor got on with his work. In the time waiting, Jack got to see a reverse history of the Doctor's regenerations. Every so often, there was a pop, a flash of light, and a different man standing, mixing chemicals. He was a fair-haired, youngish looking fellow when Jack suddenly felt horribly ill and started glowing faintly. "Doc…" he groaned.
The Doctor turned from the distillery he was working to look at Jack. "You're fine. That's just the effects of Bad Wolf. So far, you've been stuck at the same age, when she changed you, because you had lived so long. You'll start de-aging in true now." The Doctor suddenly grunted and turned away, biting his lip and wrapping both arms around his middle.
"You okay?" Jack asked, approaching to help.
"Stay back," the Doctor snapped. "You're the problem. You're still a fixed point in time… but you're also changing. It's a paradox, but a weird one. It's playing havoc on my senses." As if in agreement, the TARDIS shook slightly. "Listen, the TARDIS just moved the Zero closet to across the hall from this room. Kindly get in there before you drive her and me mad." Jack frowned, but did as the Doctor bid, leaving the room and going through the door directly in front of him.
The room was a closet, with barely enough room for Jack to stand in. Despite the cramped space, there was something calming about the room, and Jack felt muscles he didn't even know were clenched slowly relaxing. There was a slight humming noise suddenly and Jack could hear the Doctor's voice.
"Sorry, it took me a moment to remember how to access the room's speakers. Well, the almost-speakers, I should say, since there aren't any real speakers. It's really just my mental projection resonating in the room. You won't be able to respond I'm afraid." The voice sounded like the Doctor Jack knew, the skinny one with stick-up hair. He supposed it made sense, if the voice were his mental voice.
He realized the Doctor was still talking. "…fifteen minutes, half an hour max," said the voice. "Hopefully we won't be too de-aged. Sorry about the size of the Zero closet, by the way. It used to be a full room, but it got ejected a few years ago. Well, I say a few years, I really mean a few decades. Well, I mean centuries actually. The point is, I had the TARDIS start growing one as soon as the crisis was over and I remembered I lost it. But it's a complete energy null space. It's perfectly isolated from the rest of the universe, so it takes a while to grow one. I'm amazed we even have a closet already."
A few more minutes passed, and the Zero room door opened. A severe looking young man was standing in front of him, proffering a beaker filled with a fizzing liquid. "Oh dear, you've become a teenager. No matter. Here, drink this." Jack grabbed the beaker and chugged down the liquid. Unlike the first one, this one was bitter and vile to the taste.
He grimaced and asked, "So what body are you on now? You look really young, considering the general theme had seemed to be you getting older looking as you got younger."
The Doctor frowned. "I've been in my first body for a few minutes now. It's not that I've degenerated into a young body. This form started out quite old. I've de-aged to this point. You're looking at me when I was only about 25 years old."
Jack suddenly lost a foot of height. "That was a growth spurt for your information," the Doctor said.
"Wait, we're still de-aging?" Jack asked in a panic.
"Yes," the Doctor replied, pulling out a pocket mirror and inspecting his face, which Jack noticed was younger looking than before. "I'm around 18 to 20 now." Seeing Jack's panicked face, he smiled sadly. "I did the best I could. It was more important to get the solution made than to have it perfect. Hopefully what I made will be enough."
"I don't…" Jack felt another few inches disappear, "understand," he finished.
"The solution has slowed our rates of de-aging, eventually we'll stop. But it wasn't pure, so we've got a little bit more de-aging before us. Hopefully it will be before our brains have de-aged to the point where the first mixture was useless."
"What?!" Jack yelped as he felt another 'growth spurt' leave him.
"Well, if our brains de-develop, it doesn't matter how preserved our personality was, there won't be enough brain for it to remain. Cheer up, though," he smiled, "you seem to have stopped. Around eleven, I would hazard to guess," he added.
Jack pouted. He didn't like being an eleven year old again. Not to mention, there was the very big concern that an eleven year old's brain was much less complete that an adults, so it was already quite possible that he was already different.
As he was trying to figure out a test to see how immature he was now, the Doctor suddenly plummeted one and a half feet. He cursed. Judging by the lack of a translation, Jack guessed it was Gallifreyan. "That was my 14 year old growth spurt." There was another, smaller drop. "And that was my 12 year old one. I really hope this stops soon." It could have been his imagination, but Jack thought he heard a quiver in the Doctor's voice.
A few more inches disappeared, and suddenly, Jack was taller than his friend. They both waited, but after a few minutes, it was clear the Time Lord's de-aging had finally stopped as well. The Doctor pulled out his mirror again. "Hm. About eleven, I should guess. Funny how we've both ended up at the same age." The Doctor grinned and shoved the mirror into the pocket of his now too-big pants. "Come on, let's see about reversing the effects."
A/N: This is my baby. I've been writing it for a while now. I've got a nice buffer of chapters, but I'm not publishing them all at once because I am really inconsistant at writing and need said buffer. Your best bet to know my schedule for updates will be to just follow the story, since I'm not sure what my schedule will be.
This story is literally the longest thing I've ever written (research papers for school aside). I would really appreciate (critical) feedback. I hope you enjoyed the first chapter!
Sorry if you're getting an alert for this; there's no new chapter today. There was a slight grammatical error (you're ≠ your) that I could not leave alone.