"Oh, of course," Mrs. Wembley said, "the one where everyone did it."

"Are you or are you not going to tell us what the title is?" Wembley said.

"I'm not certain we should," Mrs. Wembley said. "As the Major said, it's top secret."

"But since all we're discussing is mystery novels," the horsy woman said, "you simply must read Murder in the Ca-"

"Anderson!" Patton's unmistakable voice bellowed, and everyone looked over at where he stood, riding crop raised, waving at a British officer on his way out. "Goodbye! See you in Calais!"

Ultra was decisive.

-GENERAL DWIGHT D. EISENHOWER London-November 1940 JESUS, MIKE THOUGHT, BLETCHLEY PARK. I SHOULD HAVE gone to Coventry. "You're sure Gerald didn't say Boscombe Down or Broadwell?" he asked Eileen.

"No, it was definitely Bletchley Park," Eileen said. "Why? Isn't it an airfield?"

"No," Polly said grimly.

"What is it then?"

"It's where they worked on Ultra," Mike said. And at her blank look he added, "The top-secret facility where they decoded the messages of the German Enigma machine."

"Oh, but then that's definitely where he is," Eileen said eagerly. "Decoding would be much more suited to him than the RAF, with his skill at maths and-"

"Blenheim has a park, too," Mike interrupted. "You're sure he didn't say Blenheim Park?"

"No," Polly said. "He's at Bletchley Park."

He turned on her angrily. "How do you know?"

"Because of the joke Gerald told Eileen about the rain getting her driving authorization wet. Remember? And her not being able to drive?"

"What does that have to do with Bletchley Park?"

"The driving authorization form is printed in red."

"What?"

"The bigram codebooks the German Navy used on its U-boats were printed in a special red water-soluble ink, so that if the submarine was sunk, the codes couldn't be captured."

"And?"

"And those codebooks were what they used to break the Ultra naval code at Bletchley Park."

"I can't believe this!" Mike said. "The one person who can get us out of here, and he's in goddamned Bletchley Park."

"I don't understand," Eileen said, looking upset. "Why don't you want him to be at Bletchley Park?"

"Because it's a divergence point," Polly said.

"But Dunkirk was a divergence point," Eileen said, bewildered, "and Mike went there."

"Bletchley Park isn't just a divergence point," Polly explained. "It's the divergence point. Ultra was the most critical secret of the war. It helped us sink the Bismarck and win in North Africa. And Normandy. If the Germans had had so much as an inkling that we'd cracked their codes and had access to their top-secret communications, we'd have lost the advantage that won us the war. If we were to cause that to happen-"

"But how could we? Historians can't alter events," Eileen said innocently. "Can they?"

"No," Mike said. "She just means it'll be tough to get Phipps out with all the security they're bound to have."

But as soon as he got Polly alone for a moment, he asked her, "What's happened? Did you find a discrepancy while I was gone?"

"I don't know. Marjorie-the shopgirl I worked with at Townsend Brothers and who Eileen told she worked at Padgett's-is enlisting in the Royal Army Nursing Service."

Which made no sense at all. He sat her down and made her explain it to him. When she finished, he said, "But lots of women enlisted."

"But she said she enlisted because of having been rescued from the rubble, and she wouldn't have been in the rubble if it hadn't been for me."

"You don't know that," he said. "She might have eloped even if nothing had happened to you."

"But that's not all," she said, and told him about the UXB at St. Paul's. "Mr. Dunworthy said it took three days to get it out, which means it should have been removed on Saturday, not Sunday."

"No, it shouldn't," he said, relieved that that was all. "It's not a discrepancy."

"You don't know that."

"Yes, I do. While I was looking for you, I went to St. Paul's. I figured any historian of Dunworthy's would have heard all about the cathedral from him and might show up there, and you did, just not on the same day as me. And anyway, this old guy who worked there-"

"Mr. Humphreys?" Polly said.

"Yeah, Humphreys. He gave me a tour of the whole place-sandbags and all-and told me all about the UXB. And he said it hit the night of the twelfth, which would make it three days if they got it out Sunday afternoon. So there's no discrepancy there, and lots of women eloped with enlisted men during the war. And the increase in slippage would make it harder for us to alter events, not easier."

"But if that isn't what's going on, and we can affect events-"

"Then Phipps has no business being at Bletchley Park, and the sooner we get him out of there, the better. If he's still there. If he went through just after his recon and prep, he might already have gone back."

"I don't think so," Polly said. "His joke about the water-soluble ink makes me think he's probably there to observe the cracking of the naval Enigma code, and they didn't capture U-boat 110 and get the bigram books until May of 1941."

Great, Mike thought. Phipps would have six months to louse up the war. If he hadn't already. Maybe that was why their drops wouldn't open. It wasn't something Mike had done-it was Phipps's fault.

Mike didn't say that. He just told them he intended to leave for Bletchley right away. "Shouldn't we both go?" Eileen asked. "I know what Gerald looks like. And with two of us, we'll be twice as likely to find him. We can split up-"

"No, I'm going alone."

"If it's her being conspicuous you're worried about," Polly said, "there were more women than men working at the Park. They did all the transcribing of the intercepts and ran the computers, and some of them even worked on the decoding. So if you're worried about Eileen standing out-"

That's not what I'm worried about, Mike thought. "Two people are more likely to attract attention than one," he said, "especially if they're both snooping around and asking questions."

"Mike's right," Polly said. "The people who worked there were under a good deal of surveillance." Which wasn't exactly reassuring.

"If only one of us can go, it should be me," Eileen said. "Gerald knows me. He may spot me even if I don't spot him."

Which was true. "He'll recognize me, too," Mike said, though he wasn't at all sure he would. "I need you and Polly here to go meet the retrieval team if they answer our ads. And I'll have more freedom of movement than you would. A man can go into restaurants and pubs alone without attracting attention."

"Not if you're an American," Polly said. "The Americans didn't come to Bletchley Park till February of '41. Do you think you could pass as an Englishman?"

"I am an Englishman. I had an American L-and-A, remember? But how am I supposed to pull off working there? It took clearance to get into Bletchley Park. I'd never be able to pass the background check."

"Gerald did," Eileen said.

"With carefully forged school records and letters of recommendation. That's probably what his recon trip was about, planting documents that could stand up to Bletchley Park's background check. My history wouldn't."

"You needn't actually work there," Polly said. "And by the way, it's BP or the Park, not Bletchley Park. And not Bletchley-Bletchley's the town. Bletchley Park is the Victorian manor outside of town where the decoding was done. Only a few codebreakers lived on the estate. Everyone else was billeted in Bletchley or the surrounding villages."

"Then why do I have to pretend at all? Why can't I go as a reporter and talk to them in the town, say I'm working on a story?"

"Because they've all been forbidden to talk to anyone. They've all signed the Official Secrets Act. They can get the death penalty if they talk. Besides, you'd be hauled in by the authorities instantly if they heard you were planning to write about Bletchley Park."

"I could say I was doing a story on something else," he said, but Polly was shaking her head.

"No, people will be much more likely to talk to you if they think you're one of them. If they ask what your job is, which they won't, you can say you work for the War Office. That was the official cover for intelligence work."

"How can you be so sure they won't ask me what my job is?"

"No one was allowed to discuss what they were doing. People who worked in one but didn't even know the names of the people in the other huts."

Then how am I supposed to find out if Gerald's there? he wondered. "What if Gerald's one of the people living on the estate?" he asked.

"He won't be. That was mostly the top codebreakers, like Dilly Knox and Alan Turing. Turing was Ultra's computer genius." She was looking critically at him. "You haven't any other clothes, have you?"

"No, these are the best I've got. Aren't they good enough?"

"They're too good. If you're going as a cryptanalyst-that's what they called the codebreakers-you'll have to look the part. Don't worry, we'll find you something."

The "something" turned out to be a secondhand tweed jacket with patches at the elbows, a scruffy-looking wool vest, and a tie with a large grease spot on it. "Are you sure this is what they wore, Polly?" Mike asked doubtfully.

"Positive, although the waistcoat may be too nice."

"Too nice?"

"These are physicists and mathematicians we're dealing with. Can you play chess?"

"No. Why?"

"There weren't enough cryptanalysts in England at the beginning of the war, so they recruited anyone they thought might be good at decoding-statisticians and Egyptologists and chess players. If you could play, it would make a good conversational opening."

"I could teach you," Eileen said.

"There isn't time," he said. "I want to leave tomorrow."

"No, you need to wait till Sunday," Polly said. "It'll be less conspicuous. Lots of BPers will be coming back from the weekend then. And I need to prep you."

She did, telling him everything she knew about Bletchley Park and Ultra and the principal players in such detail that he wondered if she was still worried about his altering events, too, in spite of his reassurances. She even told him what the various codebreakers looked like.

So I can keep out of their way, he thought. Which wasn't a bad idea, just in case. He memorized the names she gave him: Menzies, Welchman, Angus Wilson, Alan Turing.

"Turing's blonde, medium height, and stammers. Dilly Knox-he heads up the main team of cryptanalysts-is tall and thin and smokes a pipe. And he's absent-minded. He's been known to fill his pipe with bits of his sandwich. Oh, and he's usually surrounded by young women. Dilly's girls."

"Dilly's girls?"

"Yes. They played a vital role in the decoding. They searched through millions of lines of code, looking for patterns and anomalies."

"How do you know all this?" he asked. A horrible thought struck him. "You didn't do an assignment at Bletchley Park, did you?" If she had, and she had a deadline ...

"No," she said. "I considered it, but after I'd researched it, I decided the Blitz might be more exciting."

Not if historians can alter the course of the war, he thought.

On Sunday Polly and Eileen went to the station to see him off and to give him last-minute instructions. "The Park's in walking distance of town," Polly said, "but I don't know in which direction, and asking might look suspicious."

"I won't ask," he assured her. "I'll find a likely prospect and follow him when I get off the train."

"And I'm not sure the project's called Ultra at this point. 'Ultra' stood for ultra-top-secret, the most classified category of military secrets, and I think in 1940 the project may just have been called Enigma, and not-"

"It doesn't matter what it's called. I have no intention of mentioning Enigma or Ultra. I intend to find Gerald and get out."

"There's the boarding call," Eileen said. "Perhaps you'll be in the same compartment with someone who works there, and you can ask them if they know Gerald and how you can get in touch with him, and you won't need to go to Bletchley at all."

Jesus, he hadn't thought about running into them on the train. "What does Turing look like again?" he asked Polly.

"Blonde hair. Stammer."

"And Dilly Knox is tall and smokes a pipe."

"And has a limp like yours. And Alan Ross has a long red beard, and when it's cold wears a blue snood over it."

"Over his beard?" Mike said. "And you're worried about me being conspicuous? They sound crazy."

"Eccentric," Polly said. "Oh, and Ross has a little boy, and when he traveled, he doped him with laudanum-"

"Laudanum," Eileen said wistfully, and when they looked at her, she explained, "Sorry, I was just thinking how useful laudanum would have been on that journey to London with the Hodbins."

"Yes, well, I don't know if Ross's son was a terror or not," Polly said, "but he gave him laudanum and stowed him in the luggage rack, so if you see a little boy sleeping up in the luggage rack, you'll know that's the compartment Alan Ross is in."