A hand grabbed the back of my coverall and yanked me off my feet. At the same time there was a bright flash just to my right and a bang that was so loud it was like being slapped in the ear with a telephone directory.

I went back down – shouting. There were three more flashes, three more bangs, mercifully muffled by the water this time. I came back up spluttering and froze.

Reynolds was kneeling beside me, shoulders square and a black semiautomatic pistol held in a professional two-handed grip aimed up the sewer. Kumar was crouched down behind me, his hand on my shoulder in an effort to restrain me from leaping up and making a target of myself.

‘What the fuck are you doing?’ I asked Reynolds.

‘Returning fire,’ she said calmly.

Her pistol had one of the little back torches slung under the barrel and I followed its beam back to the intersection eight or so metres ahead. I remembered the first flash and bang.

‘Did you hit anyone?’ I asked.

‘Can’t tell,’ she said.

‘Do you know how much trouble you’re going to be in if you’ve shot someone?’ I asked.

‘You’re welcome,’ she said.

‘We can’t stay here,’ said Kumar. ‘Forward or back?’

‘If the special agent here hit someone we can’t leave them there to bleed out,’ I said. ‘So, forward.’ There was a conspicuous lack of agreement from Kumar and Reynolds. ‘But only as far as the intersection.’

‘Am I allowed to return fire?’ asked Reynolds.

‘Only if you give a warning first,’ I said.

‘What’s she going to say?’ asked Kumar. ‘Halt, totally unauthorized armed foreign national, drop your weapons and put your hands in the air?’

‘Just shout “Freeze FBI”,’ I said. ‘With a bit of luck it will confuse them.’

Nobody moved.

‘I’ll go first,’ I said.

I’m not totally mad. For one thing, the only reason I could think that our mysterious hoodie would hang about was if he’d been shot. And for the other thing, I took a deep breath and mentally ran through aer congolare – just to be on the safe side.

It was a still very cautious advance – with me in front, I might add.

The small sewer we were clambering along met a much bigger sewer at a diagonal. Judging from the yellow-brown brickwork and its relatively fresh fragrance I guessed it was a later addition and probably a floodwater relief sewer which was, judging by the water rushing through it, admirably doing what it was supposed to do.

‘Clear,’ said Reynolds and did a second three-sixty just to be on the safe side.

Upstream, the relief sewer was dead straight, vanishing off into infinity. Downstream it turned sharply down into a step-weir that dropped over three metres.

‘I think he went that way,’ said Reynolds pointing down to where the water boiled white at the bottom of the weir.