The footsteps at the lock

When Derek Burtell vanishes during a canoeing trip on the Thames, just before he's due to inherit a large sum, his insurance company sends their top investigator, Miles Bredon, to look into the case. What starts as a simple missing person inquiry quickly deepens into a complex mystery involving strange photographs, a baffling cipher, and naked footprints. Can Bredon uncover the truth behind Derek's disappearance and the subsequent vanishing of his cousin, Nigel, before the company has to pay out?