“And if you do,” Stephen said with a hint of a smile, “and I come back, I am going to haunt the hell out of you.”

We’d reached the steps of the Charing Cross Underground station, and Stephen turned to me and re-included me in the conversation.

“Here’s what you need to understand,” he said in a slightly lecturing tone. “London is one of the world’s oldest continually inhabited cities. We’ve had multiple wars, plagues, fires . . . and we keep building on top of old grave sites. Loads of buildings are built on old plague pits. The Tube system alone was responsible for disturbing thousands of graves. As far as we know, most ghosts tend to stay around the places they died, places that had some major significance in their lives, or, occasionally, the place where their body is buried. Their range varies. But the Tube has lots.”

“Lots and lots and lots,” Callum added as we reached the turnstiles.

Callum waved a pass that got him in for free. The rest of us tapped our Oyster cards, and the gates opened to admit us. I followed them to the escalators.

“The thing you have to remember,” Boo said, “is that ghosts are just people. That’s it. They aren’t scary. They aren’t out to get you”—Callum made a strange noise—“they aren’t spooky or weird, and they don’t fly around with sheets on their heads. They are just dead people who’ve gotten stuck here for a bit. They’re usually quite nice, if a little shy. Normally, they’re lonely and they like to talk, if they can.”

“If they can?”

“There’s a lot to learn,” Stephen said. “They take a lot of forms, some more corporeal than others.”

“So, who becomes a ghost? Everyone?”

“No. It’s fairly rare. From what we can tell, ghosts are people who just haven’t . . . died completely. Their death process isn’t complete, and they don’t leave.”

This I sort of understood. My parents work on a college campus, and I’d spent some time around it. Sometimes people graduate but they don’t leave. They hang around for years, for no reason. I would think of ghosts like that, I decided.

“Ghosts look like people, so you often can’t tell the difference,” Boo said. “You have the ability to see them, but it doesn’t mean you know what you’re looking at.”

“It’s like hunting,” Callum cut in.

“It is nothing like hunting.” Boo elbowed him hard. “They’re people. They look like living people, because you’re used to seeing living people. You assume everyone you see is alive. You have to consciously start separating the living from the dead. It’s tricky at first, but you get the hang of it.”

“She’s down here,” Callum said. “I saw her on the Bakerloo Line platform.”