It starts with the knife. Found at the bottom of a hidden lake in the Norfolk Broads, covered in blood and heart tissue so fresh the water hasn't yet washed it all away. What salvage diver Cam Killick has found is a murder weapon from a very recent crime – but how do you solve a murder without a body? The remoteness of the setting is itself a clue.

Only a handful of people know the location of the lake, let alone how to access it. But no sooner have Cam and DS Claire Rogers started working through this ready-made suspect pool than one of them disappears. The ripples from Cam's discovery have disturbed a dangerous predator, one who knows the water even better than Cam himself.

The question now is what they want – and how many more people will die before Cam can stop them?