‘Lukov, you fucker,’ I whispered as I ran my fingers under the card inserts. ‘Help me out.’

And so he did: my fingers closed around a soft, square, pale blue plastic packet marked Soft RIDER. The horny old bastard never went anywhere without a condom. I tore open the wrapper, put the foil in my pocket and stuffed the wallet, passport and roll of cash into the French letter. I stowed it inside my jacket, checked that all the pockets were zipped shut, and that the Glock was firmly in my waistband. The fire selector of the M4 was set to semi-auto; there was a round in the breech; the laser was on. I took a last, deep, sulphurous breath, and then my head was under. I gave the tunnel one last look with the torch beam, and went dark again.

There was no question of actually swimming. Weighed down with sodden clothes, never mind the grenades and rifle, the best I could manage was an unsteady walk. I counted out the seconds. On a good day I could free dive for five minutes. This was not a good day. Injuries, exhaustion, kit and clothing all reduced the time I’d have under water. I gave myself four minutes maximum: two minutes out and, crucially, two minutes to get back if there was no exit at the other end – and that was pushing it to the absolute limit. Once I was underwater, the most important thing was to keep moving. If I pressed on, I’d have almost perfect visibility; turn back, and I’d be blinded by my own wake. God help me if I got stuck on the return trip. When I did get out, I’d be operating in soaking wet clothes. That wasn’t ideal – but the catacombs weren’t cold. In fact, it was warmer underground than it was at the surface. The entire complex maintained a steady fifteen degrees Celsius – the perfect temperature for storing wine and for swimming assassins.