While Miss Marple and Mr Stringer are soliciting donations for a charity, they visit Mr Enderby, a rich and eccentric recluse. He tumbles down a staircase, apparently the victim of a fatal heart attack. Knowing that Enderby had a pathological fear of cats, Miss Marple becomes suspicious when she finds one in the house. She also finds a piece of mud bearing the print of a riding boot, but when she goes to Inspector Craddock, he is sceptical, believing that Enderby died of natural causes.
Undeterred, Miss Marple eavesdrops when Enderby's family gather for the reading of the Will. Each of the four heirs (fourth cousin George Crossfield, niece Rosamund Shane, nephew Hector Enderby and sister Cora Lanskenay) receives an equal share of the estate. Cora declares that she believes Enderby was murdered. The next day, when Miss Marple goes to see her, she finds Cora dead, stabbed in the back with a hatpin. Cora's companion of many years, timid Miss Milchrest (Flora Robson), can provide little information.
Miss Marple decides to take a "holiday" at the Gallop Hotel/riding school, as it is run by Hector Enderby (Robert Morley) and the other two surviving heirs are staying there. Inspector Craddock questions them (and Rosamund Shane's spendthrift husband Michael), but none of them can produce a satisfactory alibi for the time of Cora Lanskenay's death.
An attempt is made to do away with Miss Marple, but is foiled by the intended victim (without her even realising it). Miss Marple then discovers that the piece of mud found in Enderby's house came from shady art dealer George Crossfield's riding boot, but her case against him is dashed when she learns that each of the heirs visited Enderby on the day he died. Crossfield has meanwhile found out who the murderer is, but he is locked in a stall with an excitable horse and is trampled to death.
By this point, Miss Marple knows the identity and motive of the killer, but has no definite proof. She therefore lays a trap, pretending to have a heart attack at a dance at the hotel while doing the twist with Stringer. The police doctor places her in a room by herself, declaring it to be too dangerous to move her until morning. During the night, the criminal makes one last attempt to silence her, but Miss Marple is ready. The killer is revealed to be Miss Milchrest, disguised as Cora. She had been after Cora's seemingly worthless painting.
- Margaret Rutherford as Miss Marple
- Stringer Davis as Mr. Stringer
- Robert Morley as Hector Enderby
- Flora Robson as Miss Milchrest
- Charles Tingwell as Inspector Craddock
- Gordon Harris as Sergeant Bacon
- Robert Urquhart as George Crossfield
- Katya Douglas as Rosamund Shane, an heir
- James Villiers as Michael Shane, Rosamund's husband
- Noel Howlett as Mr. Trundell
- Finlay Currie as Old Enderby
- Duncan Lamont as Hillman
- Kevin Stoney as Doctor Markwell
- Frank Atkinson as Hotel Night Porter (uncredited)
- Roger Avon as Police Photographer (uncredited)