“You call a rule general if you can use it for lots of different problems or specific if it only works on a few narrow types, and you call it strong if it tells you a lot about the solution – especially if it tells you the answer right away – or weak if it only gives you a little bit. Obviously you prefer general and strong rules, but that only goes so far, at least in school. They like to give you problems that need long sequences of obscure weak rules, just to … I’m honestly not sure why. Still, you should usually start with trying the strongest rule you can think of, then work your way down.”

Bright was watching Roger impassively as he spoke, and he couldn’t tell how much of it was because she was listening to his lecture and how much she was trying to get inside his head. On one level, it didn’t really matter, in that he was paid by the hour, but it was a little disturbing. There was something about the fact that she could try to read into what he said, but he had no information on her.

“I see,” she said. She moved over to the screen and began moving abstract, wriggling geometric objects around. “This looks like the strongest rule here, but I’m not sure it applies.”

“Take the three strongest,” he suggested, “and play around with them for a bit.”

As she worked on the problem, a door opened behind them, and in walked Sue, leading Charlotte, Jason, and Blank. Sue had showered and changed into a fresh uniform; she hadn’t had a comb, so her hair was clumped and uneven, but it was the kind of obedient hair that naturally fell straight, and would be fine after it finished drip-drying.

“Sorry we’re late,” she said.

“The bell hasn’t rung yet,” Roger said. “It’s fine.”

“You’ve started early,” she said to Bright. “Since when are you a bigger nerd than me?”

“It’s not that I’m a nerd,” Bright replied, not bothering to turn around, “it’s just that I have better things to do with my life than sit around like a gormless if unusually callipygian paperweight.” She slotted a rule in, and connections tied themselves throughout the screen, forming a logical path from foundation to finish; the screen began glowing the same blue-green as the other solved door locks, and the door clicked and swung open. “Hah.”

“You think I’m callipygian?” Sue asked. “Wait, or did you mean yourself?”

“Well done,” Roger said to Bright, offering her a smile, then turned to the others. “Hi everyone. I’m Roger. I’m your new teacher.” Jason and Blank gave muted hellos; he scanned from them to Charlotte and back. “We’re all already here, so let’s keep at maths for a while longer.”

The next room had more books and stationery overflowing from shelves onto the floor. The air was dusty from disuse, making Jason sneeze, visible in the beams of light that came down through an extensive skylight. There were four more doors.

“It’s a big place,” Sue said to Roger. “We’re going to be here for a while.”

“We’re making headway already, though,” he said. “If you get flustered, you can get stuck for ages. We should really stick together; it’s easier to get stuck if you don’t have anyone to help you out.”

“We’re not supposed to,” Blank said uncertainly.

“What’s the worst that could happen?” Roger asked.

“A plane could crash,” Sue said, “rupturing its fuel tanks, which would catch fire and turn into an inferno, causing –”

“Is this going to take long?”

“Sorry.”

“No worries. You, Charlotte, and …” Roger trailed off.

“Jason.”

“And Jason, take that door. Bright and –”

“Blank.”

“Bright and Blank, take that one. I’ll show the first group how to do it, then see how you’re doing.”

Bright gave him a very piercing look. “Okay,” she said. “Come on, Blank.”

Sue and Jason led Roger and Charlotte to another door, this one with spinning cones that intersected each other, with wires spiralling down their sides like lights on a Christmas tree.

“Blank’s not wrong,” Jason said. “We are supposed to do these on our own.”

Sue touched the screen to begin. “Does it actually bother you?”

“Huh?” he asked, moving beside her to fiddle with the digital shapes. “No, I kept getting flustered and was stuck for ages, but –”

With an almighty crunch and a spray of glass shards, another two cyborg dinosaurs crashed through the skylight, masses of bright and colourful feathers, buzzsaws, rocket pods, and assorted cybernetics. They screeched, either for intimidation or from pain from all the broken glass, and turned so they were back-to-back, one facing Bright and Blank, one Roger and his group.

Jason and Sue drew their chainsaws and yanked them to start, moving forward to attack. On the opposite side of the room, Blank and Bright pulled out their weapons, Blank holding back in a practised martial arts pose, Bright going forward. The raptors dashed forward to attack, one shooting flechettes from a shoulder launcher, the other spinning up a saw grafted onto its arm.

Sue swung from the left; the raptor shied away, leaving it open to Jason to carve it apart. Behind it, Bright parried her raptor’s saw, fell back, and sliced through its neck. Both raptors fell, leaking oil and ichor, machinery falling out of wounds and spilling across the floor.

“Another benefit of sticking together,” Jason said, killing his saw. Sue handed him a tissue and he set to cleaning it.

Bright pulled out a tissue to wipe her chainsaw down, then stuffed it back into her bra and knelt down to examine the fallen raptor. Inside its police cap was an ID card.

“These were from the anti-cheating division,” she read aloud, then looked up at Roger. “We’re allowed to study together, but not work on the problems together.”

Roger had taken a flechette in his left arm. He stepped out from in front of Charlotte, pulled it out and winced, shedding a few droplets of blood. Charlotte pulled a handkerchief from her bag and pressed it to the injury, stopping the bleeding and keeping any more from getting onto his suit.

“Thanks,” he said to her, then to Bright, “They must’ve tightened up the rules in the last few years. I’ve never had the dinosaur police called on me for cheating before.”

“Mother mentioned she was trying to access new markets,” she said. “Yes, she works for Chypros. They must have signed a contract with the school, too.”

He shook his head. I really hate those guys. “Well, that’s life. We can make do. There are four doors here. One of you can work on each, with one taking a break, and I’ll –”

One touchpad turned blue-green by itself, and the door swung open. Behind was a girl of about Roger’s age, wearing digicam military fatigues. Her long brown hair was tied in a ponytail and stuffed down the back of her shirt. She had oval glasses in front of hazel eyes. In one hand, she held a badly dented golf club spattered in something oily. Her eyes flicked from Bright, who was closest to the door, to Roger, to Charlotte, and back to Roger, where they stayed.

“Hello,” she said quietly.

He blinked. “Don’t I know you from somewhere?”

“… Maybe,” she said. “I’m Lieutenant Martin, here. Call – call me Jill, please.”

“I’m Roger,” he said. “You’re not with Chypros, are you?”

She frowned. “What even is … no, I’m not. Can I talk with you, alone?”

“Er, you might want to know, that’s not a real lieutenant’s uniform,” Jason said, directing it to Roger.

Jill rolled her eyes. “I’ve had some really bad luck lately,” she said under her breath, then aloud, “I’m not a real soldier, no. And I don’t have any better weapons than this,” she waved the golf club. “So may I talk with you? It’s important.”

Roger looked over at his students, then back to her. “It’s dangerous to be on your own around here,” he said, and he took a step forward. “The raptors are out of control.”

“I think they’re deinonychus –” she said.

“We know,” said Sue.

“– but yes, I’ve noticed. Don’t worry about me. I’m pretty good at taking care of myself.”

He shrugged. Over his shoulder, he said, “Try the other problems, I’ll only be a minute,” and followed Jill into the next room, far away enough that they could talk in muted voices without being overheard but still close enough that the students could still hear that they were talking. “Who are you really?”

“Just Jill. I’m a civilian. I’m not any sort of fighter, although I like to pretend.” She looked around; there were another two unlocked doors. “I figured you’d make it around here sooner or later, and then I just followed the sound of roaring cyborg dinosaurs.”

“That doesn’t really tell me anything,” Roger said. “How did you know I’d be here? How do you know me?”

“It’s complicated,” she said seriously. “If I tell you exactly, that act will wreck something important. But it’s something like, I’m a time traveller. We’ve spent some time together in other sort-of timelines, and you’ve inherited some memories from then.”

“Not a time traveller, but something similar,” he said, thinking it over. “Timelines, with memory bleed. I would’ve thought that wouldn’t work. If I can remember anything at all, that would suggest my mind would have had to go back in time …”

“I mean it’s like time travel in the narrative sense of the same or similar events repeating,” she said. “Not in the literal sense. Your mind isn’t going back in time. But the point is, the other timelines went … badly. This one probably will too, but I’m trying to fix things. I’ve got two plans. For one of them, I need your help. You gave me half of a password, Antonio. You said he was an action hero you made up when you were a kid.”

He blinked. “I did?”

“Y-yeah,” she said, suddenly much less sure of herself. “Antonio something. I think you said you were nine?”

“Antonio … oh, right!” he exclaimed. In the previous room, all the students looked up. He lowered his voice again. “Wow, I’d forgotten about him. I told you about him? How drunk was I?”

“Um,” she said, then decided to power through this. “You told me to get you to tell me his surname, because if I told you that in the next timeline, you’d know you could trust me.”

“… Can I trust you?” he asked.

“… Yes?”

“I mean, we’ve just met,” he said, “you know a lot more about me than I know about you, including something even I’d forgotten, and you’ve given a vague and probably physically impossible explanation why. It’s literally more suspicious than if you’d said you were a stalker.”

“I’ll remember that for the next timeline,” she muttered. “Well … the reason I knew you’d wind up here is that you always do, in every timeline. See that?”

She pointed above one of the unlocked doors, at a plaque reading ‘RAYS’.

“They have a RAYS here?” Roger asked, perking up. “Nice. That might come in useful later.”

“That’s dramatically ironic,” she said. “In every timeline, one of your friends is a mole. They sabotage something, and you and your sister seek shelter in a RAYS. But it never works out.”

“What about everyone else?”

Jill shrugged. “The mole is someone different each time. It’s never been you or your sister; I’m not sure whether that’s a rule or a coincidence. So, you should stay on your guard.”

“Thanks,” he said.

“Does that mean you’ll tell me his surname?”

“No offence, but I still don’t know I can trust you,” he said. “And if I can’t, and you’d use that trust to double-cross a future timeline version of myself …”

“I guess there’s more to talking to real people than just not stuttering,” she said ruefully. “Uh, that is, I watch a lot of movies. Well … I think there’s still plenty of time, and I wanted to try something else, too. Would you help me look for someone?”

“I’m supposed to be helping these guys with their maths homework,” he pointed out.

She worried at her lip, before coming to a decision. “That might be for the best, actually,” she said. “Thank you for listening to me, anyway.” She indicated that they should go, and they went back into the room with the two dead raptors. “Hey, everyone,” she said. “Have any of you seen a man with a hunched back and a monobrow? He might have been wearing a green robe? He’s very unhelpful and taciturn, probably didn’t give a name?”

“Except for the robe, that sounds like one of the janitors,” said Blank. “I saw him lurking around the history wing earlier. I’m not sure what he was doing, except that it wasn’t cleaning.”

“That would be him,” said Jill. “Thanks.” She made for the door Roger’s group had come in through. Just before passing through, she added, “Watch your backs, all of you. I’ll see if I can’t make it back here before anything too bad happens.” Then she was gone.

“… Who was she?” asked Bright.

“I’m not really sure,” Roger said. “She seemed pretty confident something bad would happen, but she didn’t say what or why.”

“Do you believe her, Mr Abercrombie?” Jason asked. “How bad could a school day be?” Roger glanced at him; he was rooting around inside the chest cavity of one of the dead raptors, scavenging circuit boards and hydraulic pistons.

“I don’t know. There’s a RAYS up ahead, though, so at least we’re prepared.”

“You have a RAYS here?” Charlotte asked Sue, impressed.

“A better question is why it was behind a door locked with an advanced calculus problem,” Sue replied. “Who designed this place, the same guy who did the architecture in Resident Evil?”

“Should we go?” Bright asked, a little impatiently, indicating the door Jill had come from.

There were four doors on three walls: the one they saw Jill open, the one with the RAYS, another one that Jill must have opened when she first reached the room, and one beside it that was still locked. The fourth wall was taken up with French windows opening onto a huge open field, including a football pitch and a swimming pool.

“Mind if I take a look in here?” Blank asked, indicating the RAYS.

“Don’t take too long,” said Roger.

“I’ll catch you up.” She walked inside.

The rest of them trooped through the third door, which led to a food court, a hallway full of tables set with four uncomfortable metal chairs each. Around the walls were ads, more vending machines, and fast food kiosks, all but one unmanned. Roger led his class to the kiosk.

In front of it sat a man in a tuxedo with full tails, which looked very out of place on the cheap metal chair. He had light brown hair and blue eyes with smile lines; he looked about thirty but would have been shorter than Roger if he’d stood up, so he stayed in the chair. He had a hot dog half in a wrapper in front of him; judging by his expression, it hadn’t cheered him up. He didn’t say anything as they approached.

Behind the counter was a woman with dark brown hair, wearing a black cocktail dress slit most of the way up her thighs. She looked like she needed a lot more sleep than she’d had; she was leaning hard against the counter and seemed to have trouble keeping her eyes open.

“Hi, mum,” said Roger.