“A little garish for my tastes,” the old man went on. “Now. That picture has hung in pride of place since the Alte Pinakothek opened in the 1830s. They actually designed the gallery around it, because it’s so big. So you can imagine the fuss. But then, on the other hand, you have something like this . . .”

Turning to his Gladstone, he produced a rolled canvas and spread it out: a hazy oil painting of a lonely mountain pass, desolate beneath a threatening sky.

“See? Utterly different. This one was taken from a private collection in Italy. On the Way to Beziers, it’s called. From around 1880, painted by an unknown artist.”

“Wait,” Alex said, struggling to keep track. “That’s— You’ve got one of the stolen paintings?”

“Oh, ah . . . Yes. Yes, Harry managed to snatch it back off them and hide it. Just before they got him.”

“But shouldn’t you give it back? I mean, tell the police . . .” Alex felt the words draining away even as he said them.

“Tell the police?” His grandfather frowned.

“Forget I said anything.”

“Tell the police,” the old man repeated, tutting. “Honestly, Alex. Whatever’s going on here is not something the police would understand. Harry . . . Harry risked everything getting this. No. We have to get Harry back, and we can’t waste time trying to explain the situation to people who have no interest in trying to understand it. And we can’t just go giving it back until we work out why they wanted it. With a bit of luck, as long as we have this painting and they don’t, they can’t . . . Well . . . do whatever it is they’re up to.

“I can venture a couple of loose theories. One is that they’re after one particular painting and don’t yet know which, but they know it’s one of the bunch they’ve been stealing. The other possibility is they need all the paintings. The pictures add up to something, or reveal something together. There’s some connection, some hidden code or . . . I don’t know. I mean, look, take me back to all those folders again.” The old man squinted at the screen, then: “That one.”

Alex opened a folder marked CAMBRIDGE, then a picture labeled SCHALCKEN. Another painting: a young woman, blonde and pale, sitting in a dark room, holding out a plate containing what looked surprisingly like a waffle.

“Is that a waffle?” Alex asked.