“You’re the only one. I’d pulled out all the others. I didn’t realize you’d done the 1944 segment of your assignment first. That’s why you weren’t pulled out when the others were.”

“And there’s no way we’ll get out before our deadlines?”

“No,” he said. But there was no relief in his voice at having told her. Which meant there was worse to come. And if it wasn’t Colin, there was only one thing it could be.

“The reason we’re trapped,” she said, “it’s because we altered events, isn’t it?”

He nodded.

So Mike had been right.

“How did you find out?” Mr. Dunworthy asked.

“Mike—Michael—saved a soldier’s life at Dunkirk, and the soldier went back across and brought home more than five hundred others, and Michael couldn’t see how that couldn’t have caused changes, so we began looking for discrepancies.”

“And did you find them?” he asked.

“None that I could determine for certain were discrepancies,” Polly said, “but Michael wasn’t the only one who’d done something. Eileen—Merope—stopped two of her evacuees from sailing on the City of Benares, and I was responsible for a shopgirl’s being injured and nearly killed. But we didn’t know it was possible to alter the course of events. We thought the slippage kept historians from—”

Mr. Dunworthy shook his head. “We were wrong about the slippage’s function. It wasn’t a line of defense guarding against damage we might do to the continuum. It was a rearguard action against an attack that had already happened—an attempt to hold a castle whose walls had already been breached.”

“By time travel,” Polly said.

“By time travel. And in most cases over the years, the defenses were sufficient to hold the castle. But not all. It couldn’t hold it against multiple simultaneous attacks or in instances where the breach was at a particularly vital spot—”

Like Dunkirk, Polly thought. Or the autumn of 1944, when the lightest touch of a Spitfire’s wing on a V-1’s fin could change who lived and who died.

“Or in instances where the initial breach was too great,” Mr. Dunworthy was saying. “In those cases, no amount of slippage would have been sufficient to prevent the enemy from breaking through, so the only thing the continuum could do was to attempt to isolate the infected area—”

Like Eileen’s quarantine.

“—and attempt to repair the damage.”

“To shut down access to the past,” Polly said, “which is what you think the continuum did.”

He nodded. “Trapping you here.”

And you. “I’m so sorry, Mr. Dunworthy.”

He shook his head. “You are not to blame.”

“But if I’d told you I’d done the rocket attacks first,” she said. “I knew you were canceling drops and changing schedules, even if I didn’t know the reason. I was afraid you’d cancel mine, so I didn’t report in, and I made Colin promise he wouldn’t tell you.”

He nodded as if he wasn’t surprised. “Colin would do anything for you,” he said.

“Oh, this is all my fault! If I hadn’t made him promise, if I’d reported in, you wouldn’t have let me come. You wouldn’t have had to come after me—”