“I’ve got it,” the horsy woman said, looking triumphant. She turned to Ernest. “Very clever, Major, particularly the clue about trains.”

“Well?” Wembley said impatiently to her. “What is it?”

“We should have guessed it at once,” she said to Mrs. Wembley. “It’s one of her best-planned-out books, and one the reader won’t guess till the very last moment.” And when Mrs. Wembley still looked blank, “It’s set on a train, dear.”

“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 was essential to winning in the North Atlantic. And 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—”