That had helped. I’d been able to picture that one Mac computer alone on its desk with its own software, processing everything in its own way while all of the other computers, the PCs, shared their incompatible system.

But Jacqui hadn’t liked that image. “You don’t want to be alone, off in your own corner,” she had told me in decided tones. And having helped me put a name to what the problem was, she’d tackled it the way she tackled everything: head on. She’d bought me books and studied on her own, and with a single-minded focus Henry Higgins might have envied, she had tutored me in how to hide the signs, to pass for normal.

“You just have to pretend,” she’d said, choosing another analogy, “that you’re an alien, come here to learn about earthlings. Our language, our customs, our idioms, all of that. Study and learn them, the way you would any strange culture. But you don’t want to look like an alien, and that means learning to mimic. I’ll show you.”

She’d shown me. Most days, I still felt like an alien, if I was honest. But Jacqui had done her job so well these past several years that my own parents, even when faced with the facts, still refused to believe I was anything more than a little bit quirky. And in a family like mine, I thought—bringing my mind firmly back to the present as new bursts of clapping amid shrieks of laughter announced that somebody had caught the bouquet—being quirky was hardly unusual.

“How are you coping?” asked Jacqui again, and I shrugged.

“I’m all right. I could have done without the DJ.”

“Yes, well, so could we all. It was too loud for me,” she admitted, “so I can only imagine what it must have been like for you.”

My senses were…sensitive. Easily jangled and jarred. The wiring of my mind made sounds that other people could ignore strike at me with the full force of a whining dentist’s drill. Strong lighting sometimes gave me headaches, certain fabrics rubbed as painfully as sandpaper against my skin, and when all that was added to a room packed full of people, interacting in a way I had to work to understand, then staying calm became a test of my endurance.

Jacqui smiled and took a piece of paper from her handbag. “Here,” she said, and slid the paper over to me. “This might help.”

Shaking my head, I assured her, “I’m not at that stage yet.”

“What stage?”

“The Sudoku stage.” Then, because she was still watching me with that expression I’d known from my childhood, I added more firmly, “I’m fine.”

I admittedly found it a little endearing that she’d always fed my addiction to numbers, in full understanding that, when I felt overwhelmed, nothing could calm me like complex equations or, lately, Sudoku—the neat, tidy patterns of numbers in squares, like a warm, fuzzy blanket that wrapped round my mind and was instantly soothing.

It hadn’t surprised me that Jacqui had noticed when I’d made the switch to Sudoku. There wasn’t much Jacqui missed noticing. And for the past several months she had seemed to have one of the puzzles conveniently tucked in her handbag whenever I’d needed one. But…

“You can stop looking after me,” I told her. “Honestly. I’m a big girl now.”

“I know that.” Her tone told me nothing, but I’d learned that whenever her mouth tightened down at the corners like that, she was being defensive. “And anyway, that’s not a puzzle, exactly.”

I looked at the page. She was right. These were numbers, but not in an order I recognized—just numbers printed in pairs and threes, with dots between them:

106.62.181.189.68.172.766.86.128.185.64.175.

19.67.164.186.65.47.679.55.173.25.122.13.64.

562.215.128.196.29.56.63

I was already starting to look for the patterns when I asked, “What’s this?”

“It’s a code. Codes were one of your things, weren’t they?”

“When I was ten, sure.” I’d been in Year Six then. Our studies had taken us through World War II and the work of the code breakers at Bletchley Park, and I’d been so obsessed with cryptanalysis that, for the whole remainder of that winter, I had written all my school notes in a cipher of my own devising, much to the frustration of my teachers and my parents. “But that was almost twenty years ago.”

“Well, I’ll lay odds you’ve not forgotten. That code,” she said, with a nod towards the paper I was holding, “is an old one, from the early eighteenth century.”

It wasn’t actually a code, I could have told her, but a cipher. More specifically, it seemed to be a substitution cipher, in which numbers had been used in place of letters of the alphabet. But I only asked, “And why do you have it?”

“I got it from one of my authors. You’ve never met Alistair Scott, have you?”

“Who?”

“The historian, Alistair Scott. He’s quite famous. He used to be on television all the time.”

I took her word for it. I didn’t have a television. “And?” I smoothed the paper with my fingers as I focused on the numbers. There weren’t many that were higher than 500, so I guessed those might be placeholders, to mark the ends of words.

“He’s working on a new book,” she went on, “and there’s a source he needs to use, but it’s in code. He wants someone to break it for him. So I thought of you.”

“I’m hardly a professional.”

“You need the work.”