女网女高Marple is extremely proud when she is appointed to the board of trustees of a merchant marine training vessel whose mission is to rehabilitate young criminals. As in all of her endeavors, Miss Marple intends to take an active role in the job and even shows up to the board meeting dressed in naval blues. When a fellow trustee is poisoned just prior to his making some kind of revelation, Miss Marple decides to use her position to spend time on the ship. This upsets the vessel's fragile social network as she dispossesses blustering Captain Rhumstone from his quarters. When two ship's officers are later found murdered also, Miss Marple enlists help from her friend Jim Stringer and Inspector Craddock to expose the murderer.