“Then there might have been locational slippage.”

“But if there had been, it would have prevented me from colliding with the Wren.” He smiled bitterly at her. “No, I caused the slippage and then misinterpreted that cause. And we proceeded to wander through history,” he said bitterly, “gawking at wars and disasters and cathedrals, with no thought of the consequences of what we were doing.”

Polly looked at Mr. Dunworthy sitting there. Mr. Humphreys had said he looked like he had the weight of the world on his shoulders. And he does, she thought.

“For the past forty years, we’ve been blundering through the past like bulls through a china shop, fondly imagining that it was possible to do so without bringing about disaster, till it finally came crashing down on us. And on you.”

“But there was no way you could have known,” Polly said, reaching out to pat his arm.

He drew his arm back violently. “There were dozens of clues,” he said furiously, “but I didn’t want to see them. I wanted to go on believing we could insert ourselves into a chaotic system without altering its configuration, even though I knew that was impossible. That our very presence, even if we did nothing more than breathe in and out, had to change the pattern and alter the outcome.”

“But if that’s true, then we all did it, and every historian who’s ever gone to the past is to blame.” She frowned. “But why weren’t there indications up to a few months ago? Why did it take forty years?”

“That I don’t know. In a chaotic system, not all actions have significant consequences. Some are damped down by other events or absorbed or canceled out. It may have taken that long for enough changes to accumulate for a tipping point.”

Like the vases and china and crystal in the china shop, Polly thought. Each crash of the bull against the table, each pounding step, brings them nearer and nearer to the edge, till one last minor nudge takes them over it. That’s what Mike and Eileen and I did, that one last tiny nudge. And it brought the continuum crashing down.

But Mike had tried to go back through his drop before he saved Hardy’s life. Why hadn’t it let him?

“Why didn’t—?” Polly began, and realized Mr. Dunworthy was in no condition to answer any more questions. He looked dreadful, and in spite of the fire, he’d begun to shiver again.

“Time to go home,” she said. She put money down for the tea and brandy, removed her coat from his knees, and put it on.

When she took his arm, he didn’t resist, but let her lead him out of the pub, onto the wet, now-dark street and into a taxi. His hand, as she helped him in, was hot to the touch. “You’ve a fever. I think I’d better take you to hospital. St. Bart’s,” she said to the driver.

“No,” Mr. Dunworthy said, clutching her arm. “They were very kind to me. They don’t … Please, not the hospital.”