He put his hands to his head. “I wasn’t looking where I was going—an apt metaphor for the entire history of time travel. I collided with a young woman, a Wren, and knocked her bag off her shoulder, and all of her belongings spilled out and onto the pavement.” He stared blindly ahead as if he was seeing it happen. “Coins scattered everywhere, and her lipstick rolled into the gutter. She was carrying several parcels, and those flew out of her hands as well. Two other people—a naval officer and a man in a black suit—stopped to help, but it still took several minutes to gather everything up.”

“And then what?” Polly asked.

“And then the sirens went, and the Wren and the two men hurried off, and I went back to St. Paul’s Station to my drop and to Oxford.”

“And?”

“And a Wren was killed in Ave Maria Lane that night.”

“And it was the Wren you collided with?”

“I don’t know. I never knew her name. I don’t even know if she was the one I affected. It might have been the black-suited man. There’s no record of a naval officer being killed that night, so I don’t think it was him, though my delaying him might have set in motion a sequence of events which killed him the following day, or the following week.”

“But you don’t know for certain that you killed any of them, or that the collision altered anything at all.”

“That’s true. It may not have been the collision. I gave two children a shilling to tell me the name of the tube station, and had a conversation with a station guard. And I interacted with a number of other people in the station, pushing past them or making them go round me. I might have delayed any of them a critical few moments, and the difference might not have resulted in anything till much later on.”

Mike had said the same thing about the Dunkirk men he’d saved—that the alteration might be invisible for months, even years.

“In which case,” Mr. Dunworthy was saying, “it would be impossible to trace the initial altering event back to its source.”

“But from what you’ve said, you don’t know that there was an altering event at all,” Polly argued. “There’s no proof you did anything.”

“Yes, there is. Up till then there hadn’t been any slippage. It began on the very next drop. Unfortunately, that was a drop to the Battle of Trafalgar, and the one after that was to Coventry, and we drew the erroneous conclusion that the slippage made it impossible to alter events.”

“But you said you came through a day later than you were supposed to.”

Mr. Dunworthy shook his head. “I’d made an error in the coordinates. I checked it as soon as I returned. The net was set for the seventeenth.”

“What about locational slippage? You said you thought you’d gone through to Marble Arch.”

“No, I said I might have. We couldn’t do specific locations back in those days, only a general area.”