He was almost instantly sorry. As soon as the train pulled out of the station, the soldiers left the compartment to go have a smoke, and a bald, spectacled man dressed in tweeds, with a knitted vest even rattier and more full of holes than the one Eileen had found for Mike came in and sat down between Mike and the door, stretching out his legs so it was impossible for Mike to get out of the compartment without asking him to move, and he didn’t want to have any contact with him.

The man was too bald to be Turing and too short to be Knox, and he didn’t have a red beard, but he definitely worked at the Park. The moment the train left the station, he pulled out a book titled Principia Mathematica and buried his nose in it, ignoring Mike and the two ladies, who were cheerfully discussing various physical ailments.

“The pain begins in my foot and works its way all up my spine,” the one in the brown hat said. “Dr. Granholme says it’s sciatica.”

“I have a dull throbbing pain in my knees,” the other one, in a black hat with a bird on it, said. “Dr. Evers prescribed a course of nutrient baths, but it didn’t do a bit of good.”

“You should go to Dr. Sheppard in Leighton Buzzard. My friend Olive Bates says he’s wonderful with knees. I didn’t tell you, her son was called up last week. Poor Olive, she’s frightfully worried he’ll be sent somewhere dangerous.”

Like Bletchley Park, Mike thought, pretending to look out the window. BP was an exponentially more dangerous divergence point than Dunkirk because it involved a secret, and secrets were the most fragile and easily altered divergence points in the continuum. Because even though it took the combined efforts of many people to keep a secret, a single person, a single careless remark, could reveal it. Like a delayed-action bomb, which the slightest touch could set off.

All he had to do was ask the wrong question. Or too many questions. Or blow his cover. That meant he’d have to watch every word. His American L-and-A still hadn’t worn off, so he’d have to remember to keep his vowels clipped and to use the English terms for things. No “flashlights” or “elevators,” though he doubted Bletchley was a big-enough town to have elevators—correction, lifts—and it—

The train jerked to a stop. Black Hat with Bird looked nervously out the window. “Oh, dear, I do hope it’s not an air raid. I’d hoped to arrive in Bletchley before dark.”

And I’d hoped to arrive in Bletchley, period, Mike thought, hoping a passing troop train had delayed them, but they weren’t on a siding, and after a minute the guard came through apologizing for the delay and asking them to pull down the blackout blinds.

“Is it a raid?” Brown Hat asked.

“Yes, madam,” the conductor said, “but I’m certain there’s no danger.”

Except from me, Mike thought, listening for approaching planes, but nothing happened. They didn’t start up again either, and as they sat there, everything Polly’d told him about how she’d influenced the shopgirl Marjorie came back to him, and he found himself thinking about Dunkirk and all the other things he’d done besides unfouling that propeller, from tossing those gas cans overboard to hauling the dog up over the side. He’d lost his life jacket in the water. Had it floated off somewhere to entangle itself in some other propeller? And what about the body? And now here he was going to a place where a single mistake, a single word, could—