Because I saw what I had been doing wrong. The range had to be point-blank. Nothing else would work. Everything else was useless. Point-blank or nothing. Point-blank or death.

I felt the barrel of Jackson’s Glock 17 press against the rope, press so hard that I could feel the impossible tightness around my neck become even tighter.

Then I pulled the trigger.

I was aware of the crack of gunfire, a sound that seemed to rip the air apart, and then I was falling, my feet and elbows connecting with human flesh and bone.

I hit the ground hard, the gun still in my hand.

My vision was blurred with what felt like blood and tears. But I could see they were running towards a broken gap in the wall. I pointed at the back of the big man as he squeezed through the gap and screamed a hoarse curse as I pulled the trigger, my eyes streaming.

I heard the metallic click of an empty magazine.

I pulled it again and again, even after the last of them had disappeared through the hole in the wall. White noise filled my head.

I got up, spitting out a bloody scrap of synthetic material that must have come from a Nomex tactical face mask.

I stuffed the Glock down the back of my jeans, hearing a mocking voice deep inside my head.

You’re not going to shoot yourself in the arse, are you?

I took a painful breath.

And then I went after them.

I was sick to my stomach with pain and exhaustion but the rage inside was bigger than both of them. I went through the crack in the wall, stepping over scraps of rotted wood, and down the low-ceilinged tunnel until I found a stone staircase, going down even deeper into the ground. I went down the stairs slowly, moving in total blackness, afraid of falling, afraid they were waiting for me, smelling what seemed to be soot and sewers.

From somewhere I could hear the sound of heavy machinery but it became fainter as I went lower. Down and down until I reached an open space where a series of corridors met, the meeting point of a labyrinth of tunnels.

I stopped and thought I could hear voices in one of them. Then I went on, the ground always sinking beneath me. And just when I thought about turning back, when I thought I could sense the men waiting for me silently in the darkness, the stairs ended.

Ahead there were four identical tunnels, each with a rounded arch, wide but not high, built to process large numbers of people at once. They felt like they were all heading in the same direction. I carried on more carefully now, treading lightly, straining for sound.

But all I could hear was my own breathing.

And then I stepped into what looked like the train station at the end of the world.