Instead of answering, he pulls a second lever. I shrug. You'd think I'd have learned better than to ask zombies complex questions by now, wouldn't you?

The rumbling builds to a loud roar, and a remarkable object rolls out of the tunnel and screeches to a halt in the middle of the room.

It's a train, of course—three carriages, all motorized. But it's tiny. You could park it in my front hall. The roofs of the carriages barely rise waist-high, and they sport external handles. Handcart man shambles to the front carriage and hinges the roof right up. Not even breaking a sweat, he begins to load the files from his cart into a storage bin.

"Hey, what about"—I focus on the second carriage. It's got wire mesh sides, and what looks like a bench—"me?"

Handcart man lifts a box of files out of the front carriage, deposits it in his cart, and lowers the lid. Then he walks to the second carriage, lifts the roof, and looks at me expectantly.

"I was afraid you were going to say that," I mutter, and climb in. The wooden bench seat is about five centimeters above the track bed, and I have to lean backward as he drops the lid with a clang. The carriage is only big enough for a single passenger. It smells musty and dry, as if something died in here a long time ago.

Turning my head sideways, I watch as handcart man walks over to the big circuit breaker and yanks it down and up, down and up. It must be some kind of trackside signal, because a moment later I feel a motor vibrate under me, and the train starts to roll forward. I make myself lie down: it'd be a really great start to the mission to scrape my face off on the tunnel roof. And a moment later I'm off, rattling feetfirst into the darkness under London, on a false-flag mission . . .

AT ABOUT THE SAME TIME I'M FALLING FEETFIRST INTO A PIECE of railway history, another part of the plot is unfolding. Let me try to reconstruct it for you:

A red-haired woman holding a violin case is making her way along a busy high street in London. Wearing understated trousers and a slightly dated Issey Miyake top, sensible shoes, and a leather bag that's showing its age, she could be a college lecturer or a musician on her way to practice: without the interview suit, nobody's going to mistake her for an auction house employee or a civil servant. Which shows how deceptive appearances can be.