Twelve years earlier — Sunnydale, 1999.

"Okay, so it's… right hand rule," Buffy said, flipping past a vampire and staking him in the back. She adjusted her grip on her stake, so she could do the right hand rule around it. "Magnetic wire goes around like this, so my thumb points to the left. Which means that's the direction of the magnetic field."

She demonstrated by yanking her hand left, in the direction of the theoretical magnetic field of the problem — and jamming the stake into a second vampire's heart.

"Death by magnetism," Buffy commented, as the vamp dusted.

The Doctor bounced over to her. "That's it! Right hand rule! Brilliant! Absolutely…" He trailed off, as the cheer fell off his face, and he looked around himself, suddenly concerned. "Wait a tic. That's not right."

"What's not right?" Buffy readjust her grip on the stake, trying the right hand rule again, to see if she'd screwed it up. "I mean, I know I'm not all physics-geek Buffy, but… I like the hand-rule things." She flipped the stake in the air, and caught it. "They kind of go with my lifestyle."

The Doctor looked down at his hands and wrists, a look of intense concentration on his face. "It's almost like… a feeling. Like I should be restrained… or…"

The Doctor looked even more uneasy than usual. Buffy grimaced. She moved the stake to her left hand and tried the right hand rule stakeless, just to check.

A small, silver lizard scuttled out, in front of her — and Buffy lunged out and staked it with her left hand.

"So, uh… is there a left-hand rule?" Buffy asked the Doctor, trying to hide the lizard so he wouldn't notice she'd killed something.

The Doctor turned on her. "How long have I been here?"

Buffy blinked. "Huh?"

The Doctor grabbed her by the shoulders. "How long?"

"Three days," Buffy said. "One math study day and two physics study days." She shot him a grin. "I got back my math test. Guess what? Solid A."

The Doctor didn't seem to have heard her. "So this is day 3, then? Yes?" He ran a hand through his hair, pacing. "Right. Course it is. Day 3."

Buffy stepped forward to say something, then stopped. She looked a little confused. Reconsidered, then fidgeted in place.

"Look, I know I'm not the most Willow of students," Buffy put in, "but… I mean, it's just…" She shuffled, a little awkwardly. "...not a lot of people think I'm all that smart. You know?" She gestured at the Doctor. "But when you help me out, you make me feel… like… smart. Einstein-kind-of-smart." She tucked some hair behind her ears. "That means a lot to me."

"Yes, well, plenty of geniuses were rubbish at school," the Doctor muttered, taking out his sonic screwdriver and buzzing it through the air. "Michael Faraday, Isaac Newton… me, of course…" He checked the readings, hit it against the palm of his hand, then buzzed the air again. "But that's neither here nor there. Important thing is — people are still disappearing, and we've forgotten, yet again, that we should be looking for them. Remember?"

Buffy blinked. Blinked again. Then her eyes lit up. "Yes! The disappearing people!" She looked around herself, confused. "Wait, why are we here, discussing the right hand rule, when we should be out looking for the disappearing people?"

"My thoughts exactly," the Doctor agreed. He spun around, examining the graveyard. "Right! Here's the thing: I've been in this sort of situation, before. Reality changing. Minds altering. Narrative, paradoxes, all that. Only problem is, this doesn't feel the same." He smacked his lips. "Air doesn't have the right taste to it."

Buffy tried smacking her own lips, but didn't get any weird air-taste.

"Despite all that, one thing is clear," the Doctor continued. "Whoever is in charge of all this seems terribly concerned about your academic success in physics and maths."

Buffy blinked. "Huh?"

"No, no — except, it's more than that," the Doctor said, thinking aloud. He scratched his head. "Every time I'm in danger, I find myself suddenly out of it, again — and, instead, I'm back with you, teaching you physics or maths. Every time I try to investigate the disappearances, I'm stopped — and wind up teaching you physics or maths." He spun back to her. "And I've got two theories about why that is."

Buffy squashed a silver lizard underfoot, then jumped up to sit on a tombstone. "Okay. Theory me."

The Doctor raised up a finger. "One!" Gestured around himself. "We've somehow wound up trapped in some sort of educational computer game — and the people who disappear have literally been glitched out of the game."

Buffy nodded, slowly. "Okay… I don't get how, but… maybe a spell or something?"

The Doctor held up two fingers. "Two," he said. "The first night I was here, you said your mum wanted you to do better in school. She might — just might — have used the w-word around a vengeance demon, causing the two of us to be stuck in a sort of meta-universe in which all we can do is study."

"What's a vengeance demon?" Buffy asked.

"Long story; I'll explain later," the Doctor dismissed. He raised up three fingers at her. "And three—!"

"I thought you said you only had two," Buffy insisted. "When did three show up?"

The Doctor looked between her and the fingers, a little sheepishly. "It… well, it sort of… it's a bit…" the Doctor shook out his hand, then thrust three fingers at her. "Three!" he said, again, with pointed emphasis. "There's some creature out there who hasn't properly manifested in our reality — but is about to. It's gobbling people up in order to gain power and enter into this world. And it wants me to teach you physics because it's hoping I'll drop hints about one of the parts of physics that it might find terribly interesting."

"Like… what? Right-hand rule? Newton things?" Buffy asked. "Gravity stuff? Or are we talking an atom-quark-gluon demon?"

The Doctor quirked an eyebrow at her. "I was thinking more along the lines of… well… time travel. How the Hellmouth works. How all your so-called 'magic spells' actually work." He shrugged. "Take your pick."

Buffy cringed. "Okay, that actually does sound kind of likely."

The Doctor grabbed her by the hand and pulled her off the tombstone. "I think so, too!" he decided. "But whatever the answer, it's clear where we'll find it! That terribly ordinary brick wall I was looking at, last night."


It was still just a brick wall.

There was absolutely nothing special about it, whatsoever.

"Why is this thing so interesting to you?" Buffy asked him, as he studied it carefully.

The Doctor wrinkled his nose, thinking. "Not sure." He pat the wall down with his hands. "Just have the oddest feeling I've seen it before. In a rainstorm."

He yanked out the sonic and buzzed it at the wall.

The ground shook around them, as the wall exploded into a purple-blue haze, its brickwork growing fainter and fainter by the second. The Doctor kept pressing down the sonic, undeterred by the shaking of the ground or the whining scream of buildings ripping themselves apart around him.

"Doctor, stop it!" Buffy shouted, yanking him back. "Stop it, now!"

He squirmed out of her hands and popped back to his feet, still buzzing the sonic at the wall. "Oh, no," he said. "No, no, no, no, no! This is part of it. Has to be part of it. Everyone and everything keeps trying to stop me, but that just means I'm getting close! There's something about this wall that someone's trying desperately to hide!"

That was when black stuff shot out of the wall and splattered across the world. People all around them began screaming, running around in a panic. Then the black stuff spurted out of the wall again, and suddenly, people began to vanish — rapidly.

Entire crowds disappeared. Almost the whole Sunnydale downtown vanished! The buildings screamed even louder as they cracked and split and crumbled.

Buffy leapt on the Doctor's back, yanking the sonic out of his hand and flinging it away. The ground stopped shaking, the buildings stopped screaming, the black stuff stopped splattering, and the wall turned back into a wall, once more.

"What was that?!" shouted Xander's voice, as he and the other Scoobies appeared at the far end of the alleyway. "What the hell did he do?"

Buffy didn't answer.

Neither did the Doctor. Well, how could he? He had to admit, he was well and truly stumped over this one.