My foot was half over a void. I reeled back, staggered, and sat down hard on more stone. A staircase. I felt around, patting the rough stone wall, reaching down. A winding staircase, with narrow wedge-shaped steps, which meant I was in a tower. Or a dungeon.

The air had a cold, mildewy smell, which probably meant it wasn’t a dungeon. A dungeon would smell a good deal worse. But if it were a tower, light should be filtering in from a slit window somewhere above, and it wasn’t. I couldn’t see my hand in front of my face. A dungeon.

Or, I thought hopefully, the erratic bouncing had given me such a case of time-lag I’d gone completely blind.

I fumbled in my pockets for a match and struck it on the wall. No such luck. Rock walls, stone steps loomed up around me. Definitely a dungeon. Which meant I probably wasn’t in Oxford in 2018. Or 1933.

The Seventeenth Century had been big on dungeons. Ditto the Sixteenth through the Twelfth. Before that England had run mostly to pigsties and stick huts. Wonderful. Trapped in a Norman dungeon in the Middle Ages.

Or a corner of the Tower of London, in which case tourists would come trooping up the stairs in a few minutes. But somehow I didn’t think so. The steps, in the brief light of the match, had looked unworn, and when I felt along the wall, there weren’t any safety railings.

“Verity!” I shouted down into the blackness. My voice echoed off stone and silence.

I stood, bracing myself against the wall with both hands, and started carefully up, feeling for the edge of the narrow step with my foot. One step. Two. “Verity! Are you here?”

Nothing. I felt for the next step. “Verity!”

I put my foot on the step. It gave under my weight. I started to fall, flailing wildly, trying to catch myself and scraping my hand. I slid two steps and came down hard on one knee.

And if Verity was here, she had to have heard that. But I called again. “Verity!”

There was an explosion of sound, a violent flapping and whirring of wings that sounded like it was diving straight for me. Bats. Wonderful. I flung an arm I couldn’t see in front of my face.

The flapping intensified, but though my eyes strained through the darkness, I couldn’t see.

The flapping was coming straight at me. A wing brushed my arm. Wonderful. The bats couldn’t see either. I flailed at the darkness, and the flapping grew more frantic and then subsided, flying off above me, and I sat down very slowly and silently.

All right. Clearly the intelligent thing to do was sit here and wait for the net to open. And hope I wasn’t permanently stuck like Carruthers.

“And meanwhile Verity’s lost somewhere!” I shouted and was instantly sorry. The bats attacked again, and it was a good five minutes before they subsided.

I sat still and listened. Either this dungeon was completely soundproofed, or I wasn’t anywhere in the last three centuries. The world hasn’t been truly silent since the beginning of the Industrial Revolution. Even the Victorian era had had trains and steam launches to contend with, and in the cities, the rattle and clop of traffic which would shortly become a roar. And both Twentieth and Twenty-First Century have an electronic hum that’s always present. Here, now that the bats had gone back to bed, there was no sound at all.

So what now? I’d probably kill myself if I tried any more exploration, and probably miss the net’s opening in the process. Assuming it was going to open.

I felt in my pocket for another match and my watch. Half past X. Warder had had a half-hour intermittent on the drop at Muchings End and I had only been in the lab twenty minutes, in Blackwell’s possibly fifteen. Which meant the net might open at anytime. Or not at all, I thought, remembering Carruthers.

And in the meantime, what? Sit here and stare at the darkness? Worry about Verity? Try to figure out what had happened to the bishop’s bird stump?

According to Verity, detectives weren’t required to go anywhere or do anything. They could sit in an easy chair (or a dungeon) and solve the mystery just by using “the little gray cells.” And I had more than enough mysteries to occupy me: Who on earth would have wanted to steal the bishop’s bird stump? Who was Mr. C and why the bloody hell hadn’t he shown up yet? What was Finch up to? What was I doing in the middle of the Middle Ages?

But the answer to that one was obvious. Verity and I had failed, and the continuum was starting to collapse. Carruthers trapped in Coventry and then the slippage on the return drops and then Verity — I should never have let her go through. I should have realized what was happening when the net wouldn’t open. I should have realized what was going to happen when Tossie didn’t meet Mr. C.

It was one of T.J.’s worst-case Waterloo scenarios, an incongruity too devastating for the continuum to be able to fix it. “See here,” T.J. had said, pointing at the formless gray image, “and here, you have radically increased slippage, but it can’t contain the incongruity, and you can see here where the backups start to fail, and the net begins to malfunction as the course of history starts to alter.”

The course of history. Terence marries Tossie instead of Maud, and a different pilot flies the mission to Berlin, and he miscalculates the target or is hit by flak, or he thinks he hears something wrong with the engine and turns back, and the other planes, thinking he’s received orders, follow him, or because of him they get lost, the way the German pilots had two nights before, or somehow the lack of their grandson’s presence in the world affects the history of airplane development or the amount of gasoline in England or the weather. And the raid never happens.

The Luftwaffe doesn’t retaliate by bombing London. It doesn’t bomb Coventry. So there is no restoration project. And no Lady Schrapnell to send Verity back to 1888. And the paradoxes multiply and reach critical mass and the net begins to break down, trapping Carruthers in Coventry, sending me farther and farther afield. This is the cat that dropped the bomb that brought down the house that Jack built.

It was getting colder. I pulled the lapels of my blazer together, wishing it was the tweed.

But if it was a worst-case scenario, why hadn’t there been any increased slippage on Verity’s drop? “See here,” T.J. had said, bringing up sim after sim after sim, “every single incongruity has this area of radically increased slippage around the focus.” Except ours.

Nine minutes’ slippage on that first drop, between two and thirty on all the others, an average of fourteen for all drops to the Victorian era. Only two areas of increased slippage, and one of those was due to Ultra.

I took my coat off and wrapped it around me like a blanket, shivering and thinking about Ultra.

Ultra had had a system of backups, too. The first line of defense was secrecy. But if there was a breach, they put their secondary system of defenses into action, like they had done in North Africa.

They’d been using Ultra to locate and sink convoys carrying fuel oil to Rommel, which could have roused suspicions that codes were being broken, so a spotter plane had been sent up each time to be seen by the convoy, so the Nazis could blame the sinking on having been spotted.

Except once, when heavy fog kept the plane from finding the convoy, and, in their panic to make certain the oil didn’t make it to Rommel, the RAF and the Royal Navy had both shown up to sink it, and nearly blown the cover.

So the head of Ultra put a backup plan into operation, planting rumors in the port of Malta, sending an easily decodable message to a nonexistent agent, arranging for it to be intercepted. The message thanked the agent for his information on the convoy and gave him a raise. And the Nazis had spent the next six months tracking down rumors and looking for the agent. And not suspecting we had Ultra.