He would. At the end of the Blitz, there’d only be one intact window left in the entire cathedral, and that would be broken in 1944 by a V-1 that had exploded nearby, and all the replacement windows would be of clear glass.

“In the case of the altar, however,” Mr. Humphreys went on, “I’m afraid it’s another matter.”

The altar?

“Luckily, the bomb damage was confined to it and the choir.”

The choir. That’s why the volunteer at the desk had said it was closed today.

Mr. Humphreys walked across the space under the dome and to the choir. The entrance was blocked off with a sawhorse. He moved it aside and led Polly through. “And the bomb went through to the Crypt, unfortunately just at the spot where our fire watch sleeps …”

She wasn’t listening. She was staring at the choir. And the destruction beyond.

Where the altar had been was a tumbled heap of timbers and shattered stone. Polly looked up. There was a gigantic jagged hole in the ceiling. A gray tarpaulin half covered it, water dripping from its edges onto a rickety-looking scaffold beneath.

But St. Paul’s wasn’t hit, she thought, staring unseeing at the gaping hole, at the rubble. It survived the war.

“When did this happen?” she demanded.

“The morning of October tenth, just as we were making one last round of the roofs. I was—” he said, and must have seen her face. “Oh, I am sorry. I thought from what you said that you knew. I should have prepared you. It gives one a shock, I know, seeing it for the first time.”

Mr. Dunworthy hadn’t said a word about a bomb hitting the altar. He’d spoken of the UXB and the incendiaries on the twenty-ninth of December, but nothing about an HE on October tenth. “The altar was entirely destroyed, and these two windows were broken,” Mr. Humphreys explained.

“And the windows in the nave,” Polly said. It hadn’t been blast from a bomb the next street over which had broken them. It had been this bomb. Which Mr. Dunworthy had never mentioned.

“Yes. The bomb brought down more of the lower courses there.” Mr. Humphreys pointed up at the edges of the hole. “Which hit the reredos. You can see where it’s chipped, and where St. Michael’s nose was broken off.”

He went on, pointing out the damage, but she could scarcely hear him over the thudding of her heart. What if the reason Mr. Dunworthy hadn’t told her about it was because it hadn’t happened? Till now.

She’d persuaded herself there weren’t any discrepancies, that the problem was increased slippage. Which was frightening enough. But this was even worse.

This is the proof that we’ve altered events, she thought.

“How bad is the damage to the structure?” she asked, afraid of the answer.

“Dean Matthews is hopeful the underlying supports weren’t cracked,” Mr. Humphreys said worriedly, “but we won’t know till the engineers have completed their examination. The explosion lifted the roof off from end to end, and when it came down it may have damaged the supporting pillars.”