“That’s where I heard that story, of course. My father mentioned it once or twice when I was young: a puzzle he was trying to solve. I’ve been trying to remember, but I don’t think he told me much more. He was already suspicious of me by then—or, rather, he knew I was growing suspicious of him.”

“What potion?” Alex prodded, after his grandfather fell into another thoughtful silence. “What’s in it?”

“Hmmm? Oh, well, certain elements, gathered at certain times, combined in certain ways, certain words said, certain signs made. Then, y’know: fire burn and cauldron bubble. Whiz it all up, leave it to simmer with a sprig of thyme, and Bob’s your uncle. And you’ll outlive him by several generations. Unless Uncle Bob is taking it, too, of course.”

The old man fell silent, apparently studying the road, but Alex saw he was debating how much more to say. He decided staying quiet was his best bet. He tried to remember where he knew fire burn and cauldron bubble from. Shakespeare. A play they’d been reading in school. Macbeth. Witches casting spells.

“There are several ingredients,” his grandfather suddenly continued, straightening his shoulders. “Ah: magnesium, right? Pinch of that. A nip of snake venom. Some others. But those are all trifles, really.”

The old man hesitated again, clearly loath to go on. “But the essential element is a particular plant. Ah, a flower. It’s the center of it all, Alex. My father keeps it and uses an extract it produces. A ghastly flower. There’s only ever been one like it, and getting it was perilous in the extreme. Potentially fatal. Potentially worse. I was with him the night he went into the forest to take it. The Black Forest, Alex. I’ve been wary of the region ever since. A horrendous night. 1836, that would have been.”

The old man shivered at some phantom and checked his mirrors again.

“I think we should stay off the autoroute. I don’t see anyone following but pays to be careful. Be easier to tell out there.” He took the next exit off the highway. They were soon in dark countryside, steering a dim network of smaller roads. Alex’s grandfather seemed to know the way instinctively.

“As I said, Harry had been on at me to tell you about all this,” the old man resumed. “He was right. You deserve to know, Alex, there are reasons you . . . But I didn’t know how. And I suppose part of me never wanted to. I just wanted things to be normal between us as long as possible.”

Mention of Harry reminded Alex where they were heading, what he had to do. The thought of his grandfather’s friend being held in the tall man’s clutches somehow seemed even more awful now. But in turn came a memory of the strange photograph Harry had shown him.