“But you think whatever that action was, one of us did it,” Mr. Dunworthy said. “And it was one of us who won the war.”

“No,” she said, frustrated. “I’m not saying that either. No one person or thing won the war. People argue over whether it was Ultra or the evacuation from Dunkirk or Churchill’s leadership or fooling Hitler into thinking we were invading at Calais that won the war, but it wasn’t any one of them. It was all of them and a thousand, a million, other things and people. And not just soldiers and pilots and Wrens, but air-raid wardens and planespotters and debutantes and mathematicians and weekend sailors and vicars.”

“Doing their bit,” Mr. Dunworthy murmured.

“Yes. Canteen workers and ambulance drivers and ENSA chorus girls. And historians. You said no one can be in a chaotic system and not affect events. What if your—our—coming to the past added another weapon to the war, a secret weapon like the French Resistance or Fortitude South?”

“Or Ultra.”

“Yes,” Polly said. “Like Ultra. Something which operated behind the scenes, and which, combined with everything else, was enough to avert disaster, to tip the balance.”

“And win the war,” Mr. Dunworthy said softly.

There was a long silence, and then he said, almost longingly, “But there’s no proof …”

No, she thought, except that so many lives saved and so many sacrificed—so much courage, kindness, endurance, love—must count for something even in a chaotic system.

“No,” she said. “I haven’t any proof.”

There was a knock, and Eileen leaned in the door, her red hair windblown and her cheeks rosy. “What are you two doing sitting here in the dark?” she said, and switched on the light. “You look as if you could both do with some tea. I’ll put the kettle on.”

“No, wait,” Polly said. “Did you find out who the man you saved was?”

“Yes.” She took off her hat. “The admitting nurse wouldn’t tell me anything, and neither would the matron, so then I hit on the idea of going to the men’s ward and telling the nurse that Mrs. Mallowan had sent me to find out.”

“Mrs. Mallowan?” Mr. Dunworthy said.

“That’s Agatha Christie’s married name.” She unbuttoned her green coat. “The nurse and I chatted a bit about Murder in the Calais Coach, and I told her about Agatha Christie’s new book, which hasn’t come out yet. It’s all right, Polly, I told her I had an editor friend who’d let me look at it. And as a result, she let me look at the ambulance log.”

“And the man you saved was—?”

“There were three people, actually, or at any rate the nurse said she doubted they’d have survived if they hadn’t been brought immediately to hospital. I wrote them down,” Eileen said, taking a sheet of paper out of her handbag and reading from it. “Sergeant Thomas Brantley, Mrs. Jean Cuttle—that was the ambulance driver—and Captain David Westbrook.”

Mr. Dunworthy made an involuntary sound.