The being that emerged from the detector as a little wisp of cherry-flavoured smoke was so minuscule it barely counted as a demon at all. It was just a puff of power and intention, given life force by the system it inhabited, and bribed to keep the alarm from going off with offerings of flavoured smoke. Honestly, quite clever work by whoever had done it. It was amazing how creative teenagers could be when they’d thought of something that you really did not want them to do.

“Get gone, you,” Walden said, and the imp’s fragile grip on the material plane disintegrated. As the red smoke dissipated she looked around and spotted in the corner, jammed between two wire lockers, a plastic classroom chair. Someone must have dragged it down here to stand on, reach the ceiling, and draw their little summoning array.

Walden had a board pen in her skirt pocket. She could follow health and safety regulations and fetch a stepladder and a second person to watch her standing on it, or she could just get the job done.

She was tired. It was November. She dragged the chair over and stood on it. She still had to go on tiptoe to reach the detector. Eight clean, firm strokes of the board pen—it was a green one, why did she have that, she didn’t even like the green ones—and the vape detector was warded. The outline of the ward shimmered, green and gold-flecked, and then settled. It would hold for a few weeks, until Walden had time to come down here and paint up something more emphatic.

“Very nice,” said a voice behind her, and Walden startled and fell off the chair.

‘Fell’ was not the word; she stumbled, the flimsy chair went one way and she went another. The heel of one of her neat black court shoes got caught in the back of the chair. It came off her foot, and Walden went sideways. She had just enough time to think in horror, If this ends up as a broken ankle—

There was a gust of wind—leaf-scented, outdoorsy, a welcome relief from the foul fug of the basement—and Walden, rescued just in time by someone else’s spell, landed untidily and embarrassingly, princess-style, in Mark Daubery’s arms.

He grinned down at her. “You know, Sapphire,” he said, “we can’t keep meeting like this.”

“I beg your pardon,” said Walden.