The first thing I took out of my attaché case was a brown paper envelope, then I fished out a little bottle, like a bottle of ink, and a little brush, and laid them out one by one on the tea table. I then removed a document from the envelope and fanned its pages until I found a loose bit of paper. Caught between a couple of pages, it looked like a bit of scrap paper. I spun the whole thing out for as long as possible, then I laid the paper down on the tea table and showed it to him.

I wanted to have a bit of fun, so I said: ‘As you can see, this describes the person I am looking for.’

He peered at it, looked at it from a distance, picked it up, checked both sides, and put it down again. He couldn’t see anything.

‘It’s a blank bit of paper. What are you talking about?’ He looked at me suspiciously.

It was a blank bit of paper, and the only thing that might be said to set it apart from any other blank piece of paper was that it was a shade thicker than normal, and that it had not been sized, so the surface was rough.

‘No hurry,’ I said. ‘Everything that you need to know is written here.’ I opened the bottle and dipped the brush in its contents, and then started to go to work on the paper. I wasn’t writing, but laying down a thin wash, very carefully, as for a painting. Having laid down the wash the paper didn’t change colour in the least, but a fine white mist rose up and at the same time there was a very soft crackling sound – as if the paper was burning, and having been dropped into water, the flames were being extinguished.

He was amazed and asked me, ‘What are you doing?’

‘Watch carefully.’

As I spoke, the words slowly began to appear on the page, one stroke at a time, as if they were being written by an unseen hand; the order in which the strokes appeared was not that in which they were written, but when the first word appeared in its entirety, it read: ‘This’. Then the next appeared, and the next, one word after the other, as if by magic …

This document was written in a kind of invisible ink.

Why was it written in invisible ink? Of course, it was for security reasons. That way, if anything happened to me en route – say for example I was careless and lost something – even if someone else saw this paper, it wouldn’t immediately reveal my identity as a secret agent or the important mission with which I had been entrusted. My mission was to come here – to the home of Chinese mathematics – and find a genius who could crack RECOVERY for us.