“No, you don’t know the whole story,” he said, putting up his hand to stop her. “There was an increase in slippage even before you went to 1944, but it wasn’t large, and I didn’t think it was serious. The amount of slippage had often been greater than the circumstances seemed to merit, and at other times far less, and I thought there was a simpler explanation than the one Ishiwaka had arrived at, even after he showed me his equations. I certainly didn’t see any need to pull out my historians and shut down all time travel. I thought canceling the drops of historians with deadlines and putting the others in chronological order was sufficient till I had more data, but Dr. Ishiwaka was right. I should have pulled you all out.”

“But you couldn’t have known what the increase meant—”

“Dr. Ishiwaka had told me exactly what it meant, but I refused to believe him. We’d been traveling to the past for forty years without incident. I found it impossible to believe that we were a danger to the course of history. I should have listened to him. If I’d pulled you out, Michael Davies would still be alive, and you and Merope—”

“Merope?” she said, alarmed. “She doesn’t have a deadline. This was her first assignment. Wasn’t it?”

“Yes,” he said, and she knew there was still more.

“The shutdown might not be a result of the continuum’s attempt to correct itself,” he went on. “It might be some sort of reflexive response to the damage, like shock in a trauma patient. And even if it is an attempt at self-correction, there’s no guarantee it will be successful. The damage may be too great or too widespread to be repairable.”

“But it’s not,” Polly said. “We didn’t lose the war. I was at VE-Day—”

“That was before Michael saved the soldier, and you and Merope—”

“I know, but Merope was there, too. I saw her. And she hasn’t gone yet. She went there—will go there after Mike saved Hardy and we did all the other things, so they can’t have affected the outcome of the war.”

But Mr. Dunworthy was shaking his head. “At the point when you saw her, there would still have been a VE-Day to which she could go. The course of history—past and present—would have remained as it had always been until the alterations reached a tipping point. That is why we are able to be here, even though we’re part of that unaltered future. And why Eileen could have gone to VE-Day. It would have remained unaltered until the moment the final alteration occurred and the continuum could not correct for it—”

“And then everything would change.”

“Yes.”

“But you said …” She frowned, trying to grasp it. “I don’t understand. Hadn’t that tipping point already happened? The drops had stopped working.”

“Not entirely. Mine was still working in mid-December.”

“So the tipping point happened between our finding Merope and mid-December?”

“No, it may have been after that. I don’t know when exactly. I wasn’t able to get to my drop till the night after I saw you all on the steps of St. Paul’s.”