(There is a fifth, as well. The fifth is a secret you are carefully keeping from yourself. To possess Walden alive you have to be Walden; therefore you cannot think too hard about the things you are doing which she would never, ever do.)

These together make a highly secure set of footholds. Rooting you out of the world will be near-impossible without the destruction of at least one of them. Of the obvious four—body, mind, institution, machine—the body is by far the easiest to kill.

Laura knows all this. So does Todd. Neither of them has any formal qualification in academic magic, but by the estimation of demons—based on what is powerful, not what is measurable—both of them are pretty good magicians.

“I’m going for the thaumic engines,” she says. “It’ll have its claws in there. My best guess is I’ll need to take them down completely to have any chance of knocking it out of her. The school’s older defences are pegged to that key of yours, aren’t they? How long do you think they’ll hold without the modern stuff?”

Todd sucks his teeth. “Hard to say, hard to say. Depends who’s holding it. Well.” He lifts the key on its chain off its peg by the fireplace and drapes it round Laura’s neck. “There you go.”

She frowns. Todd is already retrieving, from its place of honour on the table by the front door, his personal toolbox. It is a battered heavy-duty black case with a faded set of stickers on the side—Pink Floyd, Led Zeppelin, AC/DC, Motörhead—and a lovingly polished steel handle. It represents an investment of years of time, money, and expertise. There is a much more extensive array of tools in the workshop conversion at the back of this old cottage, but in here lies the core, the heart, the river-channel his magic flows through. These things to him are keys to power, much as salt and chalk and the principles of arcane annotation are keys for you.

“You’ve never set foot in that engine room, Marshal,” he says, pre-empting Laura’s protest. “You can’t just go in and start smashing things willy-nilly. You’ll never do enough damage before she gets there. That old key looks after itself, mostly. You’ve got the knack, haven’t you? Just let it lead you along and it’ll tell you where you need to be.”

“Mr Cartwright,” says Laura, “I can’t let you put yourself in danger this way.”

The old man snorts. “Girl half my age tries to stop me from doing my job,” he says. “There are kids here, you know.”

…