Ash was happily floating and I wondered if I was the only one finding it hard to maintain a conversation while treading water. Something warm brushed against my leg and I twisted in time to see a girl pop her head out of the water. With just the lights from the bank she was hard to see clearly, but I recognized the cat’s curve to the corner of her eyes and her mother’s strong chin.

“What are you? Lifeguards?” I asked.

“Not exactly,” she said. “If you make it out of the river under your own steam, fair enough. If you don’t then you belong to Mama.”

The first girl surfaced again and rose out of the water until she was waist-deep and as steady as if she were standing on a box. I noticed she was wearing a black wet suit with ORCA written across her chest. Enough light caught her face for me to recognize her as Olympia, aka Counter’s Creek, one of the younger daughters of Mama Thames, which meant that the other was no doubt her twin sister, Chelsea.

“Do you like the suit?” Olympia asked. “Neoprene. It’s the best you can buy.”

“I thought you guys liked to skinny-dip?” I said. Their older sister Beverley had swum naked the last time I’d seen her in the water.

“In your dreams,” said Olympia.

Chelsea surfaced on the far side of Ash. “I thought I smelled blood,” she said. “How you doing, Ash?”

“Much better now,” he said drowsily.

“I think we need to get him back to Mama,” she said.

“He told me to get him in the river,” I said. My legs were getting really tired and I looked around to find the shore a lot farther away—I was being dragged out into the central channel.

“What do you want—a medal?” asked Chelsea.

“How about a tow back to shore,” I said.

“Doesn’t work like that,” said Olympia.

“But don’t worry,” said Chelsea. “If you go under for the third time—we’ll be waiting for you.”

And then, with an unremarkable plopping sound, they vanished under the water.

I swore at some length at that point and would have sworn for longer except I was freezing to death. I tried to gauge which bank was closer. It was tricky because the combination of the tide and current was sweeping me toward Black-friars Bridge. The same bridge under which Roberto Calvi, God’s own banker, got his neck stretched—not really a promising omen for me. I was freezing and trying to remember the water survival training I did when I got my swimming certificate in primary school. My legs felt heavy and my arms ached and, as far as I could see, neither bank was closer.

It’s remarkably easy to die in the Thames; lots of people manage it every year. I was beginning to worry I was going to be one.