Head-lamped and silhouetted, rifle hanging by his side, the boy’s assailant reared up as I emerged from the water. I thrust the barrel of the carbine past the boy’s head and into the man’s throat. I fired. The first round blew his windpipe out. The second emptied his brainpan into the void of the tunnel. The boy hit the floor to my left with a thud, rasping.

I turned. The dry tunnel was less than two metres wide. A torch blinded me. Then two rifle reports deafened me. The first round skimmed my chest. The second ricocheted off the brickwork behind me, bouncing back to clip the upper receiver of the M4. I dropped into the water. I swept the suppressor across the attacker; he moved his barrel across me. We aligned. I fired first. His head jerked back. His rifle went down. I got up and out of the water. He was braced against the wall, left hand on his stomach, screeching through gritted teeth. I crouched. Gun up, breathing hard. I’d gut-shot him. I tensed on the trigger again, but wanted – needed – to get something, anything, from him before he died.

I switched the LED torch on next to my forward grip and blinked the water out of my eyes. He wasn’t feeling his stomach for an entry wound; he was struggling to free the cotter pin on a grenade hanging from his webbing.

I squeezed the trigger, but it would not pull.

Stoppage.

His legs went out from under him. As he fell, the grenade rolled clear, the ring hooked in the crook of his finger. I dropped the rifle and twisted right. I hit the half-drowned boy in the lower back as he struggled to stand, wrapping my arms around him, rolling us into the depths of the flood. I pushed us under. My ears rang from the shots. I felt my heartbeat.

And then: boom.

The blast wave pulsed through the water, through us, as we spun thrashing in the darkness. The expanse of the tunnel absorbed the force of the explosion, but still I felt it. I held the boy fast, getting his head above the water and then levering us both over the lip of the pool we’d tumbled back down into.

I sat up on the floor, coughing, while the boy crawled on all fours over the body of the first man I’d shot seconds earlier. He vomited water, and then whatever else was in his stomach. I retrieved my M4, its torch still on, and travelled the beam around the tunnel. I saw the boy properly for the first time. He wasn’t a teenager but a young man. Five-ten; a hundred and thirty-five pounds; hair covered completely by tightly wound black cloth. He was wearing blue jeans tucked into wellington boots, and a green T-shirt. He tried to stand, but collapsed back to his knees.