Dear Philip: I work in a blinking madhouse, Osla imagined scribbling to her fair-haired prince—not that she could give him details about her new job in those letters posted to Philip’s ship, but she’d got into the habit of talking to him in her head, spinning the straw of daily life into entertaining gold anecdotes. It’s a small madhouse tucked inside a larger one. The large one is Bletchley Park, the small one is Hut 4. Hut 4 simply defies description.

She’d turned up for her first shift promptly at nine the morning after signing the Official Secrets Act, thrilled to her bones to be doing something more important than pot-riveting seams. All she wanted in this world was to prove herself, prove once and for all that a Mayfair giggler who’d curtsied to the king in pearls and plumes could poker up in wartime and serve as well as anyone else. Could do something important, even . . .

Well, banging Hurricanes together might have been useful, but this was in a different class. Osla had already vowed she’d stick it out here, no matter how hard it was. She was only sorry she and Mab wouldn’t be working together. Dear Philip: The girl I’m billeted with is simply divine, and I forbid you to ever meet her because you would probably fall in love on the spot and then I would have to hate her. Not you—you wouldn’t be able to help yourself; Mab would wing a superb eyebrow at you and that would be that—but I can’t afford to hate her because it’s clear I will need allies if I am to survive in the house of the Dread Mrs. Finch. More about her later.

Osla and Mab had sauntered to the gates of Bletchley Park in the bright June morning, where Mab was shunted to Hut 6 and Osla to Hut 4. “Well then . . .” Mab perched her little chip hat at an aggressively chic angle. “Show me just one eligible bachelor, Hut 6, and we’ll get along fine.”

Osla hoped Mab was met by a more appetizing specimen than the fellow who answered her own knock: a stocky balding fellow in a Fair Isle jumper. “German naval section,” he greeted Osla as she stepped into the long green-painted building squatting next to the mansion like a frog. “You’ve got the German, then?”

“You mean have I got a German tucked in my handbag?” Osla quipped. “’Fraid not, darling.”

He looked blank. She sighed, spouting some Schiller in her impeccable Hochdeutsch. He waved for her to stop. “Good, good. You’ll assist with the registration, the W/T sorting, the teleprinted traffic . . .”

He whisked her inside the hut and showed her through: two large rooms separated by a door, a small room at the end, another little room after that which had been subdivided even smaller. Long tables heaped with papers and atlases, swivel chairs, pigeonholes, green steel filing cabinets . . . it was stiflingly hot, the men in shirtsleeves while the women patted perspiring faces with handkerchiefs. With a distracted “Have a go!” he passed Osla over to a motherly middle-aged woman who took in the new arrival’s evident confusion with a smile.

“It wouldn’t be any clearer if he tried to explain. These Oxbridge types are hopeless at explaining anything.”

Dear Philip: My entire introduction to the world of codebreaking was “Have a go!”

The middle-aged woman introduced herself as Miss Senyard and made introductions to the others—a few girls like Osla, all Mayfair diction and pearls; a few girls with “university” stamped all over them, all efficient and friendly as they showed the new girl the ropes. Some were sorting wireless telegraphy forms; some were collecting unknown German naval codes and identifying call signs and frequencies with slashes of a pencil. Osla received a towering stack of loose papers and a punching machine—“Take these signals and bind them up properly, dear. It’s the early naval Enigma traffic; poor Mr. Birch’s cupboards are positively overflowing and we’ve got to get it filed.”

Osla studied a sheet: a report of some kind, translated German broken and patchy as if parts of the sentence hadn’t come through. “Why is this in German and not that?” she asked the girl next to her, nodding at the cards with their keys and call signs, much of it gibberish.

“This is the undeciphered stuff. We log it, register it, then it goes out to the naval section boffins to be broken. The boffins are the brainy ones.” Admiringly. “Who knows what they do or how they do it, but the undeciphered stuff comes back to us broken into readable German.”

“Oh.” That was where the important work was done, then. Osla wrestled with the punching machine, fighting a sense of deflation. Punching holes to bind papers together and stick them in cupboards—was this really the best use of her language skills? Had she managed yet again to land in a place where the real work was being done by someone else? Not that she was going to get in a wax about needing to be important, she just wanted to be used well . . .

Never mind that, she scolded herself. It’s all important. And it’s only your first day. “What do we do with all these reports and signals, then? Once they come back broken into German.”

“It’s all translated, logged, analyzed. Miss Senyard’s box files have copies of every German naval and naval air signal—periodically we get someone in a tearing hurry, requesting a copy of this report or that one. And we send the raw decrypts to the Admiralty, as well as reporting by telephone. We’ve got a direct line; Hinsley rings since he’s liaison, then they give him the brush-off and he goes about muttering insults for the next hour.”

“Why do they brush him off?”

“Would you believe it if some reedy Cambridge student from the middle of nowhere called to tell you where the U-boat wolf packs were, and when you asked how that information was obtained, his reply was You don’t need to know?”

Dear Philip: The Admiralty currently making decisions for your beloved navy lurches along on shrugs, shoeboxes, and ignorance. Is this entire war run by idiots? That would explain why we’re on the verge of being invaded. Not that she ever would have written Philip anything so defeatist. Osla kept her letters cheerful; the last thing a man at war needed was gloom from the home front. But to herself, in her own head, she didn’t mind being pessimistic. It got difficult keeping your chin up, all the while imagining what London would look like once the Jerries had nailed German street names over the signs to Piccadilly and St. John’s Wood. It could happen. Not that anyone said it, but everyone was in a pelter worrying that it would happen.

The Americans weren’t coming to the rescue. Most of Europe had fallen. England was next. That was the bleak reality.

I might see the news here first, Osla thought, reaching for a new report. She might know before anyone else in the country—before Churchill, before the king—when they were being invaded, because the next decoded German report might be orders for a pack of destroyers to sail for Dover. Just because the brainy boys here could decode what the Nazis said to each other, that didn’t mean they could stop it.

I don’t know what you’re doing in there, Osla thought to the boffins breaking codes for the U-boat packs that hunted ships like Philip’s, but do it faster.

That made her wonder. “If this is naval section, can we look up our own ships in the decrypted reports? See if the Germans have flagged them in their radio traffic?” Like HMS Kent, currently bearing a certain fair-haired royal midshipman toward Bombay . . . “Or are we not allowed to ask about such things?” The orders had been no talking to anyone outside Bletchley, and no talking to anyone outside or inside about one’s work, but those instructions still left quite a few gray areas. Osla had no intention of breaking the Official Secrets Act on her very first day. Dear Philip: I’m going to be hanged for treason, or possibly shot by firing squad.

“We all talk in-hut,” the reassuring answer came. “It’s all right as long as everything you learn stays in-hut. You can try looking up a ship if you’ve got a fellow on board, but you can’t pass on anything you find to his mum.”

That wouldn’t be a problem, Osla reflected. Philip never mentioned his mother. He’d talk about his sisters, the ones who had married Nazis and to whom he could no longer write; he’d talk about the sister who died in a plane crash with her entire family a few years ago; he’d even mention his long-estranged father—but never his mother.