It was more than a decade ago. Walden felt only nostalgic fondness for Roz now, tempered by the knowledge that neither of them had behaved very well during the long slow-motion breakup that had culminated in Walden fleeing back to the UK without telling her girlfriend the plan.

It was very strange to see Roz at Chetwood School. In the weak January light she made the whole place look small and grey and unconvincing, and Walden’s best Open Day patter somehow only succeeded in turning her world and her life into a minor tourist attraction. Roz exclaimed over the fourteenth-century chapel and colonnade, marvelled at the grounds, looked up with admiration at School House’s silly Victorian tower. “It’s just so cute,” she said with conviction. “I can’t believe how cute this is. I can’t believe you live here.”

“Thank you,” said Walden, and handed Roz off with some relief to Philomela Jones, who looked monstrously pleased to have someone new to tell all about Lady Margaret Beaufort. Meanwhile she went to check with ICT that everything was properly set up for the talk over in the new building.

She had her fingers crossed before it started. Roz had been only an indifferent teacher, had resented lecturing because it took her out of the lab. Years Eleven, Twelve, and Thirteen were packed onto every inch of space on the tiered seating in the school’s only lecture theatre, with some unfortunate Year Elevens even forced to sit on the floor. Lunch had been a while ago. A big, sleepy, potentially tricky crowd.

“Who are the ones in the red sweaters?” said Roz, when Philomela dropped her off, wearing the slightly startled look of a person who had been on the receiving end of aggressive American charm.

“The red jumpers? Oh, that’s Year Eleven. Er, sophomores,” Walden said. “I think. They still have to wear uniform.”

“All right,” said Roz. “Wish me luck.” And then, completely bypassing the formal introduction Walden had meant to give her, she scooped the microphone off the lectern and said to the crowded lecture theatre, “Hi! Can you hear me?”

Walden shouldn’t have worried. Of course Roz hadn’t stood still for a decade. She was no longer the newly minted Dr Chan, all prickles and ambition and tunnel vision. Somewhere along the line she’d picked up public speaking. It was a perfectly learnable skill, and one thing Roz had always done spectacularly well was learning. She had most of the mob of teenagers eating out of her hand after about five minutes. They were naturally inclined to find her interesting. After all, she was a living answer to the utility question: What can you do with magic? And she also answered the other question, usually unspoken but always hovering over this crowd: What is magic going to do for me?