Peace in her eyes, Dot shook her head.

“Well then.” Margot smiled and reached into her bag for the handful of almonds Maman had tucked in there after Margot had made both their sandwiches. She handed a few to Dot and popped the others into her mouth. “It looks as though we might as well be friends.”

Dot munched on her nuts and didn’t say a word. Just smiled a little and relaxed against the bench.

Margot did the same. This could work. Which she had pretty much known the moment the Pythagorean theorem spun through her head upon spotting her.

Mathematics never led her astray.

4

Margot.”

She looked up, numbers still clouding her eyes until she blinked. Admiral Hall came into focus. He stood at her side, irritation on his face and a paper in his hand, which he slid onto her desk. “Would you? It’s from Thoroton. I’ve been called in to a meeting with the First Sea Lord.”

He looked none too happy about it, that was for sure. But then, she knew he had not been pleased with how Jellicoe had been running things of late.

Her gaze dropped to the paper. With a glance, she saw that it was one of their own telegrams, not an intercepted one, encrypted in the code Hall used with his agents and operatives in the field. All it would take to decode it would be to pull out the book he’d slipped her a year ago for occasions such as this and perhaps twenty minutes of deciphering. “Of course. Shall I just put it on your desk when I’m finished?”

“Yes, that will do.” He patted her shoulder. “How is the latest German codebook working for us?”

“Quite well.” The cryptographers in Room 40 could break the new variations day by day—that was simply a matter of finding a new starting place in an existing pattern. But to unravel the pattern itself was a different story, and the Germans were constantly changing their codebooks. Oh, they could crack the codes by sheer mathematics—eventually. They’d done it with Code 13040. And while that might be more sporting, it also took more time. Months, years more time. And they hadn’t that leisure.

Hence why DID offered prizes to divers who could recover German codebooks from U-boat wreckage. That was where this latest one had come from, which she was now using to render a message sent last night to UC-44, a German minelayer. Putting Thoroton’s telegram aside for now, she tapped the page on which she was writing the translation. “I could be wrong, sir, but . . . but so far this reads rather knowledgeably. ‘Sweeper to be in harbor at midnight on Tuesday. Postpone operations until Wednesday and then proceed. . . .’”