Our doddering investigator Miss Jane Marple (Margaret Rutherford) visits the rich recluse Mr. Enderby (Finaly Currie) on routine charity work, when the old man suddenly topples from the head of his staircase, dead of a heart attack. Miss Marple becomes suspicious of a cat in the Enderby mansion: she happens to know the old man was deathly afraid of them. Further snooping reveals that Enderby's estate is to be split equally among four heirs, one of whom, Cora, suspects foul play and immediately turns up with a fatal hatpin in the spine. Cora's longtime companion Miss Milchrest (Flora Robson) is no help. Miss Marple turns next to The Gallop, the riding school run by Cora's nephew Hector Enderby (Robert Morley), where two of the other heirs are staying. One of the two, the art expert Crossfield (Robert Urquhart) uncovers the plot but is locked up in the paddock and trampled. Miss Marple lays a trap for the murderer, pretending to exert herself by dancing the Twist in the hotel ballroom, faking a heart attack, and advertising her vulnerable condition. The killer pays a midnight visit to her room and is dramatically unmasked.