I swept my light along the path but the suspects – assuming that’s what they were – had scarpered. Nightingale seemed more interested in the river than the path. I used my torch to check the water around the narrowboat which, I saw, was drifting slowly downstream, but there was nobody in the water.

‘Shouldn’t we check there’s no one left on board?’ I asked.

‘There had better be no one on that boat,’ said Nightingale loudly, as if speaking to the river rather than to me. ‘And I want that fire put out right now,’ he said.

I heard a giggle out in the darkness. I pointed my torch in the direction it came from but there was nothing to see except the boats moored on the far bank. I turned back to see the burning boat being sucked down into the river as if someone had grabbed hold of the bottom and yanked it under the surface. The last of the flames guttered out and then, like an escaping rubber duck, it bobbed up to the surface, the fire entirely doused.

‘What did that?’ I asked.

‘River spirits,’ said Nightingale. ‘Stay here while I check further up the bank.’

I heard another laugh from across the water. Then, very clearly and not three metres from where I was standing, someone, definitely a woman and a Londoner, said, ‘Oh, shit!’ Then came the sound of metal being torn.

I ran over. At that point the bank was a muddy slope held together with tree roots and bits of stone reinforcement. As I got close I heard a splash, and got my torch on it just in time to see a sleek curved shape vanish beneath the surface. I might have thought it was an otter, if I was stupid enough to think otters were hairless and grew as big as a man. Just below my feet was a square cage made out of chickenwire, part of an anti-erosion project I learned later, one side of which had been torn open.

Nightingale returned empty-handed and said that we might as well wait for the fire boat to come and take the remains of the narrowboat under tow. I asked him if there was such a thing as mermaids.

‘That wasn’t a mermaid,’ he said.

‘So there are such things as mermaids,’ I said.

‘Focus, Peter,’ he said. ‘One thing at a time.’

‘Was that a river spirit?’ I asked.

‘Genii locorum,’ he said. ‘The spirit of a place, a goddess of the river, if you like.’ Although not the Goddess of Thames herself, Nightingale explained, because her taking a direct part in any aggro would be a violation of the agreement. I asked whether this was the same agreement as ‘the agreement’, or a different agreement entirely.

‘There are a number of agreements,’ said Nightingale. ‘A great deal of what we do is making sure everyone keeps to them.’

‘There’s a goddess of the river,’ I said.