Elderly sleuth Miss Marple takes to the seas to investigate murder on a naval training ship.

At a board meeting of the Cape of Good Hope Youth Reclamation Trust, trustee Cecil Ffolly-Hardwicke drops dead before he can reveal some vital information concerning H. M. S. Battledore , the naval cadet ship used by the trust to rehabilitate juvenile delinquents. Miss Marple, a fellow trustee and private detective, discovers that Hardwicke has been poisoned, though the police discount her theory. To the dismay of Captain de Courcy Rhumstone, she boards the ship to continue her investigation, leaving her trusted assistant, Mr. Stringer, on shore to aid her. While Miss Marple searches for clues to the murder, Lieutenant Compton, discovered by the detective instructing a group of cadets in the profitable sideline of housebreaking, and Shirley Boston, an assistant matron, are murdered. Miss Marple learns that Commander Breeze-Connington has been embezzling funds for a number of years and has committed the murders in order to protect himself. She confronts Breeze-Connington; a fencing match ensues; and Miss Marple holds the murderer off until the police arrive.