The Thames River Police accompany developer Harry Exeter to a derelict waterside district to exchange a ransom for his kidnapped wife. The operation becomes an ambush, leaving men dead and convincing Commander Monk that someone within his own force revealed the plan.
Monk investigates the histories and loyalties of officers he has trusted while Hester examines the financial and domestic power surrounding the Exeter marriage. Solving the kidnapping requires him to identify both the external conspirators and the betrayal that allowed them to attack.