When a fishing-boat owner, Harry Tremaine, is killed in the explosion of his fishing boat, suspicion falls on the man's son, who stands to inherit everything, but Wycliffe remains unconvinced. There's also a local developer who wants to build a marina on the quayside Tremaine owned; Tremaine's partner has been having an affair with his wife; and there is a mysterious recipient of £500 in cash from Tremaine on the first day of every month.