“I think these are extra dials,” Jane said. “You turn the dials to change the way you scramble the letters. If you put different dials in, you get even more ways to mix them up. This will keep us busy!”

I saw that you could set up the machine to turn your message into nonsense, and then someone with the same machine, or one like it, could decode your nonsense by using the same setting.

A German coding machine was sitting on my bed. Did Robert Ethan know what Felix Baer had brought with him? He hadn’t said anything about it.

But he’d definitely been looking for something.

I knew we’d have to tell someone, but I didn’t know where to start. It must be terribly, terribly secret. We couldn’t telephone anybody—the people at the exchange might hear anything we said.

“What’s that you’ve got?” Jane asked me.

“I think it is the key that goes in the corner. The pilot must have taken it off on purpose—he left it with the wishing coins this morning. He said my name. I think he wanted me to find it, but I don’t know why.”

I showed her.

I poked the nail of my little finger into the hollow back of the key, wondering why it didn’t fit over its peg. Something was jammed in there, but even my little fingernail was too big to tease it out.

“Use a hairpin,” suggested Jane. So I did.

The jammed-in thing was a curled-up strip of paper. I unrolled it carefully and smoothed it out against the tea tray. It had a name written on it in pencil: Django Reinhardt.

We stared.

Jane turned one of the dials slowly. I looked over her shoulder and tried to do the maths in my head to work out how many combinations you could use to set the dials. Thousands of millions. No, I must be calculating wrong. Millions of millions of different ways to set the code.

It would be so easy to do with the machine and just about impossible to undo without it.

“Go fetch the Django Reinhardt records,” said Jane. “Bring both albums.”

“But that Intelligence officer emptied all the record sleeves,” I reminded her.

“Get them anyway.”

Mrs. Campbell did no more than glance at me as I came racing back downstairs for an armful of Jane’s records. I felt like I’d been running up and down the dark stairs of the Limehouse carrying record albums all my life.

I got back to Room Five, breathless, and dumped the records on the bed. Jane opened the first album. “There’s a separate pocket in here for album notes,” she explained. “That’s where I used to hide my bank passbook.” She checked, found nothing, and picked up the other record.