We continued towards the hotel in silence. I tried to ignore the chill in the air, struggling to work out what all the slaughter amounted to. Baaz’s fingers drummed the air beside him as he walked. Whatever was going through his mind didn’t make it to his lips. Speaking slowed him down. Numbers energized him. I made my own calculations, too.

Whatever Rachel was working on was being underwritten by the GRU, lubricated with a slick of untraceable filthy lucre.

The visit to Gallery 7 answered the question I’d asked Moshe: I now knew which Russians were supplying the cash. But if Rachel hadn’t simply absconded, Moscow could still have been responsible for her disappearance. Just because the doctor and his goons were behind the curve didn’t mean the Russians were out of the picture. If Leonid Avilov had exceeded his orders as far as I’d exceeded mine, he could be a long way off-piste – and as out in the cold as I was.

I considered that carefully. It was a real possibility: my father – every inch the Ministry of Defence loyalist as well as a brilliant scientist – had once told me that ‘London is not a monolith’. Thames House competed with, and loathed, Vauxhall Cross; MI5 and MI6 agents outplayed each other for grace and favour, ego and advancement; government undermined the military; the Foreign Office undermined everyone else. They all played their own games for their own ends – ends which sometimes, but not always, excused the means. There was no reason to expect that Moscow was any different, and every reason to suspect it wasn’t.

In the catacombs Avilov had told me that I was a hard man to help. Maybe he wasn’t the enemy. Or maybe that was the point: there were no enemies, because there were no friends. Whoever held the banknote held the cards. Perhaps Rachel knew she’d been dealt a dead man’s hand and folded. The more I looked at it, the more that hundred-dollar bill looked like aces backed with eights – with the queen of hearts in the hole. I thought of Doc’s last words to me: She’tamut. To death. It was a game that had already seen the end of a better gambler than me.

The closer we got to the hotel, the clearer it became: there was only one way to protect Baaz. Rachel had unburdened herself. It was suddenly inevitable I’d have to do the same. We drew up alongside Independence Park, deserted in the winter night.

‘Stop a minute,’ I said. ‘I’ve something for you. Give me your hand.’

Baaz hesitated and then stretched out his palm, his fingers finally stilled. On to it I pressed the folded banknote.

‘Don’t you need it?’