Knox reached down, pinched the corner of the thick, off-white paper and turned it over. His name was written on the front and, from the weight of it, there was a letter inside.

He walked back to the kitchen counter, gently deposited the envelope on it, and thought about what to do. What he should do was cover it in something airtight, douse his hands, and call Leconfield House. But his instincts told him he wasn’t under attack. He didn’t think the envelope had been dipped in cyanide or filled with anthrax spores. Whoever the letter’s author was, they wanted to make contact with him, not kill him.

Still, he needed to be cautious, so he used a knife to prise open the glued-down flap and tip out what was inside. Not a letter, but a card. It was marked in the same ink and handwriting as the envelope, and spelled out a simple message: Regent’s Park Broad Walk, Midday, Old friends.

Adrenaline rushed through Knox. It was exhilarating, but it was also so long since he’d felt it that for a moment he wasn’t sure what it was. He had no idea who these ‘old friends’ could be, but he wanted to know.

He put the card back into the envelope, slipped it into his jacket, and rushed out of the flat.

After yesterday’s excitement, Leconfield House seemed even quieter than usual when Knox arrived twenty minutes later. He thought about checking in on Laing, but decided he should probably see if the younger officer had sent anything new about the Kosygin summit up to his office before he bothered him again. He also considered taking his note to White and getting him to check for any invisible messages or clues about who had sent it, but realised he wanted to keep his mystery to himself.

He found a fresh pile of folders on his desk. Laing was continuing to be as thorough as he’d claimed, and was making a point of letting Knox know. It took him most of the morning to read through the files Laing had compiled, followed by the latest communications on how poorly things were going in Rhodesia.