‘What’s up?’ Roth said. Ben made no reply. He walked over to the door and tried the handle. It was locked. He crouched down beside it and picked up one of the other leaves, and felt the gentle breath of a draught against his fingers.

‘There’s something behind this door,’ he said.

It took both of them to shoulder it open with a crack of splintering wood. They stepped through the doorway into a concrete space some ten feet square, and for the first time in nearly two hours tasted fresh, cool air. The floor was covered in leaves.

Ben craned his neck upwards as another dead leaf spiralled down like a snowflake to meet him. High above, through the iron grid where the ventilation shaft opened up at street level, he could see stars twinkling. A metal ladder with safety hoops ran part-way up the wall to a grille platform, then another, and a third section extended all the way to the top. Alongside the ladder was a cable pulley system with an electric hoist for lifting equipment up and down the shaft inside a steel cage.

‘There’s your other way in,’ he said to Roth. ‘And our way out.’

‘Well, I’ll be damned. Those wily sons of bitches.’

The hoist system might have looked ancient and ropey, but it was a hell of a lot preferable to trekking all the way back through the tunnels. The big red Bakelite switch on the cage’s rusty old control box still worked fine. Ben fetched Margot Segal from the inner room, and moments later the three of them were riding up the shaft as the cables creaked and the pulleys whirred. The hoist came to a juddering halt at the top landing, just a few ladder rungs below street level. They breathed in the fresh breeze from the ventilation grid, and could hear the sound of occasional passing traffic. Nearly four in the morning, Paris was at its quietest, oblivious of the battles that had been taking place below ground that night.

The grid was hinged like a trapdoor but secured by a heavy padlock, which offered little resistance to a couple of silenced gunshots before they dropped its mangled remains back down the shaft. Ben heaved the grid open, tossed his bag with the guns inside up onto the pavement, and pushed his head and shoulders out. The ventilation shaft opened up onto an alley off a narrow tree-lined side street he didn’t recognise. The adjoining main street was just a few yards away. He pulled himself out of the shaft and ran over to it, and realised that by luck they were just a couple of hundred yards from where they’d left the car. He ran back to the shaft and helped Margot Segal clamber out. Roth was the last to emerge.

‘Well, that was fun and games.’