“Do you know who Captain Westbrook is?” Polly asked him.

Mr. Dunworthy nodded. “He was killed on D-Day, after single-handedly holding a critical crossroads till reinforcements arrived.”

For there is nothing lost that may not be found, if sought.

—EDMUND SPENSER, THE FAERIE QUEENE

London—Spring 1941

“SO YOU’RE TELLING ME ALF AND BINNIE ARE WAR HEROES?” Eileen said after Polly and Mr. Dunworthy had explained Polly’s theory to her.

“Yes,” Polly said. “You were right about their being a secret weapon. Only they’re on our side. Their jumping out in front of you when you were chasing John Bartholomew and delaying you was what was responsible for your being forced into driving the ambulance that night, so that you were able to save Captain Westbrook’s life—”

“And they delayed the train.”

“Train?” Polly said.

“When we came to London. They chased a headmistress out of our compartment, and she tried to have us thrown off the train, and it made us late leaving the station. And later we found out the railway bridge ahead of us had been bombed, and Alf said, ‘It was a good thing we was late.’ ” She looked up at Polly wonderingly. “They saved my life. And the headmistress’s.”

“And you saved Captain Westbrook’s.”

“And you two and Mike and I won the war?” Eileen said.

“Helped to win the war,” Mr. Dunworthy said. “Tipped the balance.”

“But I don’t understand. If they’d lost the war before we came, then how could you have been at VE-Day? There wouldn’t have been a VE-Day, would there?”

“Yes,” Polly said, “because by 1945, you’d already saved Captain Westbrook’s life and I’d already saved Sir Godfrey—”

“But you hadn’t done that when you were at VE-Day,” Eileen said, hopelessly confused. “You hadn’t even come to the Blitz yet.”

“Yes, I had,” Polly said patiently. “I came to the Blitz in 1940, and I went to Trafalgar Square on VE-Day five years later, in 1945.”

“But what about all those years before any of us came here, before time travel was even invented? The war was lost then, wasn’t it?”

“No,” Polly said. “It was always won because we had always come. We were always here. We were always a part of it.”

“The past and the future are both part of a single continuum,” Mr. Dunworthy said, and launched into a long and involved explanation of chaos theory.

“But I still don’t understand—”

“Don’t understand what?” Binnie asked, coming in and announcing that from now on she wished to be called Florence—“Like Florence Nightingale”—and become a nurse, which put an end to the conversation.

But the next morning after Alf and Binnie had gone to school, Eileen brought up the subject again. “So because Mr. Dunworthy ran into the Wren and Mike untangled the propeller and you saved Sir Godfrey, it changed things just enough that we won the war, is that right?”

“Yes,” Polly said.

“Then there’s no reason to keep us here,” she said, “and we can go home.”