The Doctor is kept in a vault below the city of London.

But it's the wrong incarnation of the Doctor.

This is the Fifth Doctor. And he keeps nattering away to himself, content and yet bustling around, inside his little tiny prison. When Willow lets Seo and Dawn inside, the Doctor notices Willow.

He bows to Willow, immediately meek and cowering before her.

"The Doctor," Willow announces, to Seo and Dawn. "Our very own pet Time Lord." She locks Seo and Dawn inside, with the Doctor. And leaves. "Enjoy."

The Doctor is completely different, personality-wise.

He's clearly still vastly intelligent, and the vengeance demons are obviously using that intelligence for their own ends by making him do complex computations for them — but he's very much a 'go-with-the-flow' kind of guy.

The Doctor sees no point in opposing the Vengeance Demons.

They've won, they're in control, and what's the point in opposing people in a position of authority?

"I tried it once, in my youth," the Doctor explains to Seo and Dawn. "Decided to defy the Time Lords, and stick my nose into things. I wound up having to steal a TARDIS and run away from Gallifrey with my granddaughter."

Seo and Dawn are both aware of this.

"But I was caught," the Doctor explains. "Placed on trial. They found me guilty of breaking the 'Don't interfere' law, and sent me for... rehabilitation."

And now it becomes clear what's changed in him.

And why he's only in his fifth incarnation.

"You never left Gallifrey, after that," Seo realizes. "The Time Lords convinced you to become... like this."

"Naturally," the Doctor says. Then, as he sees their faces, adds, "But ultimately, it was for the best! I mean, just think about it! I was out of control. Always questioning authority, trying to defy the status quo. If I'd kept up behavior like that, continued wandering across time and space as a renegade… think how many regenerations I might have gone through, by now!"

Twelve, Seo and Dawn both figure.

"No, best leave the fiddling and the futzing to the CIA," the Doctor concludes. "I've learned that the best thing to do is not to speak out or fight back against authority, but to simply do as I'm told."

Dawn is completely horrified at what the vengeance demons have done to the Doctor.

"They must have wanted someone who knew about Gallifreyan technology," Seo guesses, "and could lead them through how it worked and operated. They changed Mom... to alter the crucial paradox. And change the Doctor."

They soon learn that, in this version of history, the war had ended because Braxiatel's plan had worked (except that Gallifrey had not been rewritten; the plan had only wiped out the Daleks). The residual energy that lingered had transferred to Buffy, who had become a shallow person who went along with peer pressure and didn't care about the fate of people.

The paradox had been altered.

Along with time itself.

The Doctor had been remapped into a new person, who went along with authority and didn't question. He had no memory of ever being anyone else.

(Brax's plan worked because, in this reality, the Doctor never met Benny Summerfield. The Dynadum never turned into a race of super-advanced beings. And Brax's plan with the energy field was a success.)

(The Doctor probably explains this to Dawn and Seo, in the present, by telling them about Brax's plan, in the past, and remarking that Brax was very surprised that the energy field was still around. Brax had told the Doctor that he believed that "time has changed", and that it had something to do with the Doctor and Bernice Summerfield. The Doctor had protested that he's never met a Bernice Summerfield, and Brax had said, "That's what I meant.")

Anyways.

Taking control of the paradox that caused Buffy and the Doctor to be the way they were is too big a job for just vengeance demons. Seo surmises that the only way the Vengeance Demons were able to gain control of it and change it was by using the Moment.

They'd said that the Moment had been useful.

Useful, indeed.

"So that's it," says Dawn. "The Doctor's the only person who could help me get back my children, and the demons have already rewritten history to turn him into their slave!"

The Doctor says that he resents the implication that he's a slave. He's simply trying to make the best of a bad situation.

"I'm terribly sorry about your children, of course," says the Doctor, going back to his work for the vengeance demons. He starts tinkering. "If there's anything I can do to help find them..."

Seo cuts in, suddenly. "How'd you cross universes?"

The Doctor looks up, confused. "What?"

"You must have been off Gallifrey when the catastrophe happened," Seo reasons. "In fact, you can't have even been in the same universe as Gallifrey. Otherwise, you'd have been killed with all the others."

"Yes, that's right!" Dawn realizes. "The disaster killed all Time Lords in the same universe as Gallifrey! So if the Doctor from this timeline didn't move Gallifrey to another universe, then why is he still alive, when everyone else is dead? That doesn't make sense."

The Doctor ponders this, because they're right, it doesn't. "I suppose..." Then he stops. Scratches his head. Not sure how to answer this.

"I think the Moment's been holding a number of impossibilities at bay," says Seo, "to make sure the Doctor is still around and alive in this universe, at this time. And that's strange. After all, the vengeance demons have certainly gone through a lot of effort to get Father this way — but why?"

"Because he's a Time Lord," Dawn says. "And knows the technology."

"But there are other Time Lords they could have used and rescued who'd have caved, instantly, without needing to rewrite any timelines at all!" Seo argues. "They can obviously use the Moment to pluck any individual Time Lord out of the way of the dimensional disaster. So why did they choose Father, specifically?"

Then, realizing something else, turns on the Doctor.

"And where's your TARDIS?" Seo asks.

Now the Doctor gets indignant. "You're not who you say you are!" he accuses. "You're one of those vengeance demons! All this talk of changed timelines and lost children... just to make me confess something I don't know!" He gets very angry. "I've told you before, and I'll tell you again. I don't know where the TARDIS is! I've never known!"

Dawn exchanges a look with Seo.

"The Vengeance Demons have been trying to make him tell them where the TARDIS is?" Dawn asks.

Seo nods. She'd guessed as much. "That's why they didn't kill him," she said. "That's why they changed him into someone who'd tell them anything they wanted to know. To find out where his TARDIS was."

Dawn doesn't get it.

"I've been stupid — thinking of time as just one dimension, like a human," Seo says. "As if cause and effect are simple. As if the Vengeance Demons came into power, and therefore, to revenge themselves on me, they altered the paradox to change Mom and Father. But time's not just one dimension. It's got more complexity than that. Especially when people are changing history."

The Doctor sees what she means, instantly.

Dawn doesn't. "Huh?"

The Doctor explains it. Imagine a situation in which cause leads to effect. "You're doing chores, mopping the kitchen floor, but you leave the floor wet, because you're hungry and have decided to break for lunch," the Doctor explains. "You make yourself a jam sandwich, but before you can eat it, you slip on the floor and drop it. The sandwich winds up getting swallowed by the dog, and… no lunch."

"Okay."

"But now imagine that you really wanted that jam sandwich," the Doctor continues. "So you build yourself a time machine, and go back to that moment and dry the floor, so you didn't slip and drop it."

"Okay," says Dawn.

"The cause for you drying the floor would be because you'd seen a future in which you hadn't," the Doctor says. "It isn't caused by anything in a one-dimensional time continuum. If you hadn't seen the future, you'd never have dried the floor in the first place."

"And if you couldn't cross to that point in time," Seo prods, "because you don't dare touch something else that you want to make sure remains constant?"

"Then you go back further, and change something else — to make yourself want to dry that floor, in your own past!" the Doctor continues. "Perhaps... you go back into your childhood, and make yourself become obsessed with drying floors. Or you..."

He stops.

Suddenly worried.

"Oh, dear," says the Doctor. "I'm beginning to see where you're going with this."

"I don't," says Dawn.

"The reason the Doctor's alive," says Seo, "even though time says Gallifrey should never have been moved and he should have died with everyone else... is because this is the Doctor from the old continuity — the one who moved Gallifrey into another universe and won the Time War. He survived while everyone else died, simply because Gallifrey was in one universe, and he was in another."

"But that's not what he remembers!" Dawn insists.

"Of course not — because they wanted a jam sandwich," mutters the Doctor. "So they reached back into my past... and changed who I was."

Dawn still doesn't get it.

"They figured out the Gallifreyan technology by themselves, Aunt Dawn," Seo explains. "Then came back to Earth and started using their powers to take over the universe. But imagine, if that had happened... what the effect would have been? If Father had never been changed."

"The Doctor — the old, normal Doctor — would have figured out what the vengeance demons had done," says Dawn, "and he would have come here, to Earth, to stop them."

"By…?" Seo prompts.

"By… finding and tracking down the one person who knew about vengeance demons," Dawn realizes. "Buffy! The unaltered, normal Buffy!"

Seo agrees.

"I think... Mom and Father came up with a plan, together, to take down the vengeance demons," says Seo. "Something to do with the TARDIS. Father must have sent it away, when the Vengeance Demons found it out. And no matter how much the Vengeance Demons tried, they couldn't get its location out of him."

"So they went back in time," the Doctor mutters, "and changed me into the sort of person... who'd tell them."

"They'd probably have had a hard time interfering with Gallifrey's past, without changing the events that led to them getting into power in the first place," Seo agrees, "but... they'd figured out that Father and Mom are linked. Change Mom, before she jumps into the Portal..."

"And you change the Doctor!" Dawn realizes. "That's why Willow wasn't allowed to interfere with Buffy. Because they were using Buffy to reshape the Doctor!"

Seo nods, again.

"But, of course, what they failed to realize," the Doctor concludes, "is that, by changing me, they've now turned me into someone... who'd never have sent my TARDIS away in the first place. So I don't know where it is."

"But it's still gone," Dawn says.

"Yes," says the Doctor, pondering over this. "The other-me must have worked out some way to make that stick, even in the eventuality that they changed my own timeline."

Seo reminds Dawn of what Willow had said — that D'Hoffryn wouldn't be happy to let Seo and Dawn visit the Doctor.

"Whatever my parents figured out, whatever plan they made to defeat the vengeance demons," says Seo, "it's something both of them knew about. And something... that you and I also know about, Aunt Dawn. Something that happened when Mom and Father were together on some other adventure. D'Hoffryn doesn't want us to see the Doctor, because he's afraid that you and I might work out what their plan was, and take him down."

(This would probably make more sense if D'Hoffryn caught Seo and Dawn with the Doctor, earlier, and they're having this whole conversation with the Doctor while they're on the run from D'Hoffryn.)

"Or perhaps someone I once met," the Doctor proposes, "whom I didn't, anymore. A companion, or..."

Suddenly, Dawn punches Seo. Hard.

"You!" Dawn shouts. "I'm going to kill you, Seo!"

The Doctor struggles to separate Dawn from Seo, who has no idea what the hell is going on. She thinks maybe Dawn's been rewritten by the vengeance demons.

But that's not it at all.

"Someone they both met, you said!" Dawn shouts. "Oh, yeah, it all makes sense, now! What's the one thing that remained the same, before and after the Vengeance Demons took over! What's the real reason the Time Lords wanted to wipe me out of time?!"

"Dawn, what are you...?"

"You told me it was okay!" Dawn screams. "You told me it wouldn't hurt the baby! I thought you knew something, Seo. Something that made this not like Amy Pond and the Silence. But you didn't know anything! You were wrong!"

Neither the Doctor nor Seo know what Dawn's talking about.

But Dawn, suddenly sapped of strength, tumbles to the ground. Sobbing.

"You killed my children, Seo!" Dawn says. "You killed my children!"