Nikki looked thoughtful. “Isn’t it weird that it’s never happened before?” she said. “You’d think, if the engine’s been there since the nineteenth century, and we’ve had demons hanging around Chetwood that whole time … if there’s an obvious problem, wouldn’t it have come up already?”

“That is one of the big questions, yes,” Walden said. “There simply isn’t a natural point of failure. Or rather, all the natural points of failure were spotted long ago, because they were obvious. So we dealt with them long ago, and now none of them seem to be the problem. When you know there’s something like Old Faithful lurking, you take warding extra seriously. And I take warding seriously anyway. I could have sworn that every defence on the school site was being maintained to the very highest standard. But clearly, somehow, they’re not.”

“Maybe someone damaged the protections on purpose, then,” Nikki said. “Somewhere hard to find. Like that lesson where you rewrote the incursion ward before we came in, and none of us noticed till Matty did the safety check.”

“That’s hardly likely, Nikki,” Walden said.

“Sorry,” said Nikki, but she looked thoughtful. “It’s only because I just revised this. Initial magician error, check the alerts and redo the wards. Natural failure over time, check the alerts and redo the wards. Protection not strong enough for the local magical environment, check the alerts and redo the wards. And if it’s not any of those, then someone’s invited something in, so check the alerts and redo the wards and then call the Marshals. Right?”

“Textbook,” said Walden. “Full marks. But I do think a bad actor is the least likely explanation. That’s what you’re proposing, you realise, a magician willing and able to undermine Chetwood’s magical defences on purpose. It would have to be someone with internal access to the school site, and they would have to be very, very skilled—not just a student messing around. I did consider ‘student messing around’ as a possible explanation, but believe me, I would have caught that by now. So that only leaves, for your bad actor hypothesis, an adult magician with considerable magical skill, remarkable subtlety, and no morals whatsoever.”

“And that wouldn’t happen here, would it?” Nikki said. Her dark eyes were distant and thoughtful. Walden couldn’t tell if she was sincere or not when she said softly, “Not at Chetwood.”

When Nikki was gone, Walden did a desultory review of her workbook for the arcane safety carousel. She’d got it into very good shape years ago, but it paid off sometimes to stop and think whether your standard lesson plan was still working. And her brain needed a break from the puzzle of the thaumic engines. This at least felt like a productive way to change gears.