Tasks had a way of piling up. Little things you needed to go back and do and then you just forgot until one day, fixing something old and insistent, you found a misplaced post-it note and hit your head when you sat up in a panic.
Predestination, perhaps. Something he'd always vaguely suspected and (a decades-old memory) the reason he'd first slipped those specs into the pocket of a likely-looking suit.
One week and no more. Easy enough to drop Rose at her mother's and pretend a need to sort something out (not entirely pretended, but presented as a vague obligation that named no names).
This is, he thinks, why you should never trust people who have time machines.
In 1982, Rose Tyler is not yet a twinkle in her mother's eye.
In 1982, Dorothy McShane (later to be known simply as 'Ace') is twelve years old and in need of a substitute teacher who will, as it happens, be summarily dismissed after teaching a class of first year students how to make explosives from ingredients they might find lying around at home.
So in 1982, the Doctor becomes, for a week, a chemistry teacher.
"We had this chemistry teacher," Ace had explained, "Mr Smith. He was only there for a week, but he made it all seem really interesting. They sacked him because he taught us about gunpowder and someone blew up one of the local strays."
He'd been shocked by that. "He taught you about explosives? In a high school?"
She, of course, had thought of this as wonderful. "Yeah. Not just that stuff, but that was the bit anyone remembered. Shame it wasn't on the curriculum or we'd all have done really well that year."
"Well," he'd said, "at least you got some use out of it all." Which had made him wonder, just a bit. "What was he like?" (He'd been ever so good at seeming casual, back then.)
"He wore the same suit every day and he had these Elvis Costello glasses." She'd made a little glasses-gesture with her hands. "Sometimes you remind me of him a bit. If you looked completely different and sounded different and you were a chemistry teacher. So nothing like you at all, really. Except he was. A bit."
He'd shrugged it off as coincidence until Fenric, when he started to wonder about the merits of planning ahead even more than he already did.
At least it's only chemistry. That shouldn't be too difficult. He'd taught Leela to read fairly fluently inside two (relative) years, and he was quite determined to take a bit of the credit for that, thanks very much.
Ace is only twelve, and not his Ace. She is still called Dorothy, sits at the back next to Manisha Purkayastha and writes things on the desk. She has so many wounds that he can't heal yet in case she never gets to Iceworld.
He doesn't play favourites, but Ace has always felt like family, a comfortable click in conversation. It's mutual and she starts answering questions.
"You could be good at this if you actually did the homework," he tells her.
She shrugs and manages to give the impression that she won't.
She will, though. She will.
Manisha will die. Manisha has to die. Manisha is already dead.
This is the omnipresent temptation of the time-traveller. How easy it would be to ensure that the Purkayastha family were simply not home that night. To call the police a few hours before some kids throw a petrol bomb through a window.
If Manisha does not die, then Ace never burns down Gabriel Chase, never tells the Doctor how Manisha died, never gives him this foreknowledge. Simple paradox, and these days it would be enough to destroy everything.
Manisha will die. She is twelve years old, and has only a year left.
Ace's first attempt at liking chemistry goes quite well. The Doctor watches like a proud parent as the liquid in the test tube changes colour and reaches the correct pH.
Ace's second attempt shatters glass, burns the bench and almost takes her hand off. Near-injury aside, the Doctor is even happier with this result.
"I didn't mean to break anything," she says, sweeping up glass as she should almost certainly not be allowed to. (It's okay, they're firing him on Friday anyway, after a cat gets blown up.)
"Yes, you did," he says, flipping though a textbook to see if it might hold her attention.
"I just wanted to see what would happen."
"I didn't say you shouldn't have done it." He drops the book onto the bench and sits on a stool, leaning on his elbows as she empties broken glass into the bin. "Ace," he says. (No, not yet.) "Dorothy."
He's not sure. She's just a kid and he already knows what things would only upset her. She is not his Ace, not for years yet.
He pushes the textbook across to her. "You should take this. Just have a look," he says, before she can protest. "You can always set fire to it or something if you hate it."
"I don't read much," she shrugs.
"It's got pictures," he says, flipping the pages for her. "And the ending's a surprise. I'd never have guessed if I hadn't already read it."
Ace sniffs and takes the book. "I suppose."
"Wicked," he says.
On Wednesday, Ace mixes lead nitrate and potassium iodide to produce a cloud of lead iodide. It's not dangerous, but it's impressive and it proves she's opened the book.
It proves a few things.
Thursday arrives, and he is halfway to the TARDIS when he hears what has to be a cat being blown apart. Building site, just round the corner, and the noise that comes afterwards is the one that sets him running.
He's heard Ace cry before (more than once).
Ace is intact (of course she is), and there is soot on her face, streaking down her cheeks. "I didn't mean to. I was only trying to scare it."
"You need containment," he says, tired. "Otherwise you can't control the blast. You need... a box or something. Cans." He sits next to her on a pile of rocks. "Are you alright? You didn't get hurt or anything?"
"My mum'll kill me," she says, broken up.
Oh, Ace, he can't say.
"Go home," he tells her. "Or go round Manisha's and get yourself cleaned up."
"It's not your fault, Dorothy. You have to stop blaming yourself for everything."
She stands up and glares at him, angry. "It was you that told me how to do it! What did you do that for? I'll get expelled and then my mum'll murder me and then what?"
He shakes his head. "Nah, you don't get in trouble for finding a dead cat."
She gives him her stare, the one when she's trying to figure out how crazy you are and if she likes it. She's his Ace, just waiting on her catalysts.
"Piss off, then," he says. "Someone's probably called the police after all that racket."
She takes a step away, uncertain.
"And work on containment," he tells her. "Control it. Keep it in. You'll get it right in the end, Dorothy. Just have a little faith, yeah?"
She stands there, silent, thinking, dust swirling round her feet. "You should go home too," she says, "in case the filth work out it was you that taught us how to make that stuff."
"Yeah," he nods, "you're right." He watches her leave, feet kicking the dirt. "You'll have other teachers," he mutters in her wake.
He sits in the dust and waits.