Walden started marking her sixth formers’ mocks on Monday morning at 10 A.M., and had the grades in the system half an hour after the deadline. A* for Nikki, A for Will and Aneeta, a low C for Mathias, all more or less as she’d expected. She knew her late turn-in wouldn’t be the last. Annie in the office would be sending chasing emails until Wednesday. When it was done, she stretched, groaned, and went straight back into the cavernous room where the thaumic engines lived.

Todd hadn’t left. For once he’d taken off the heavy chain he wore around his neck with the Great Key of Chetwood hanging on it, and left it on the polished table by the door into Walden’s rooms. Now he lay in almost the same position Walden had left him in two hours ago, though he’d moved over to the left: flat on his back with torch and screwdriver under the brass-and-polished-walnut edifice that controlled the roadside boundaries of the school wardings. There was a pried-off panel at his side, and a double handful of ancient screws in a Tupperware takeout container. Most of them, Todd said, needed to be replaced. They hadn’t found the hole in the wards yet, but the 1960s expansion of the school to take in the building site for Scrubs had required a matching expansion of the school’s magical protections. Walden thought it was as likely a weak spot as any.

“Anything?” she said, without much hope.

“Not a brass monkey’s fart,” said Todd.

He’d been in here since half past four this morning. Walden hadn’t started until half six. I’m an old man, I don’t sleep much, he’d said. The hole’s in here somewhere. The Key feels it. I can feel it.

Walden had a lot of respect for Todd’s magical feelings, but she hadn’t found any obvious problems with the thaumic engines’ inner workings yet, and she couldn’t sense anything wrong. Neither could the Phoenix. And Todd was rapidly approaching retirement age. Nine hours from a 4 A.M. start was a long time for a person in his sixties to spend wrestling with ancient machinery, muttering curses to himself the whole time. “Take a break. Go and get some lunch.”

“With respect, Dr Walden—”

“It’s Saffy,” said Walden. “We’ve worked together for years, Todd, please. I know you know Chetwood inside out. I do need your help here. I’m asking you to have a rest and something to eat and come back fresh. I think we’re in for another late night.”