As I ran down the ramp, I heard the Incident Response Vehicles skidding to a halt behind me and the medley of grunts, shouts, and radio chatter that announces that the Old Bill is here to sort you out. As I ran across the width of the pier something whacked me hard across the thighs. The safety railing I realized too late, and I went headfirst into the Thames.

The Goddess of the River will proudly tell you that the Thames is officially the cleanest industrial river in Europe, but it is not so clean that you want to drink it. I came up spitting with a metallic taste in my mouth.

A dark shape bobbed in the water a yard from me—Ash floating on his back.

I wear a pair of Dr. Martens shoes for general detective work. They’re smart, hard-wearing, and, crucially, retain some of that horrorshow goodness for kicking that still makes DMs the footwear of choice for all right-thinking skinheads and soccer hooligans. On the other hand, they’re heavy and you do not want to be wearing them while treading water. Once I had them off, I splashed forward to check on Ash—he appeared to be a lot more buoyant than I was. I could hear him breathing and it sounded stronger than before.

“Ash,” I said. “You feeling better?”

“Much better,” he said languidly. “The water’s a bit salty but nice and warm.”

It was bloody freezing for me. I looked back at the pier to see my fellow policemen shining their torches across the water but it was okay, because the tide was still going out and Ash and I were already a couple of hundred yards downstream. Well, okay until we were both swept out into the North Sea or I died of hypothermia or drowned—or most likely an exciting combination of all three.

The current took us under the arches of Waterloo Bridge.

“You never told me she was a pale lady,” said Ash.

“Who’s the pale lady?” I asked.

“Lady of death,” he said, and then added something in a language that sounded a bit like Welsh but probably wasn’t.

“Hey,” said a nearby voice. “What are you doing in the river?” Young, female, middle-class but with the clipped vowel sounds that comes from having parents who believe in education or else. This would be one of Mama Thames’s girls.

“That’s a difficult question,” I sputtered. “I was driving home from Oxford, Ash called me, and it all went pear-shaped from there. What are you doing in the river?”

“It’s our turn on the rota,” said a second voice as we emerged on the other side of the bridge.