Wapping, London: March 1799. A sunken lugger in the Pool of London at last reveals its shocking secret. River Surveyor Tom Pascoe of the Marine Police finds the bodies of two men entombed in the crew's cabin. Suspicion falls on a third member of the crew seen fleeing the scene. He had a known motive for murder. Against the background of a nation at war with Napoleon, Tom Pascoe finds his own life under threat as he digs deeper into the case. He uncovers the existence of French agents whose task is to undermine England's ability to continue the war. In a race against time, Tom's job is further complicated by the presence of a new member of the police crew with a secret of his own...