The wise old snoop Miss Jane Marple (Margaret Rutherford) joins a Trust dedicated to rehabilitating delinquents. The Trust has recently purchased an old battleship, the HMS Battledore, but matters turn dark when she witnesses the death of a fellow Trustee who has just visited the old ship. She finds that his snuff has been poisoned. Miss Marple resolves to visit the ship herself, to the dismay of the police captain Rhumstone (Lionel Jeffries). Two more mysterious shipboard murders follow: one by both sword and hanging, another by poisoned mousetrap. To set the trap for the murderer Miss Marple announces that she's spending the night aboardship alone, while two secret policemen lie concealed. Her discovery of a concealed trove of cash leads to a confrontation with the murderer, which in turn leads to an entertaining fencing duel before the final resolution.