The film changes both the action and the characters. The original novel featured Hercule Poirot rather than Miss Marple, and Christie's trademark suspense is seasoned with light comedy. Miss Gilchrist from the original novel has also morphed into Miss Milchrest.
Plot
While Miss Marple and Mr Stringer are soliciting donations for a charity ("The Reformed Criminals Assistance League"), they call on Mr Enderby, a rich recluse. He tumbles down the long entrance 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. There are four beneficiaries: fourth cousin George Crossfield, niece Rosamund Shane, nephew Hector Enderby and sister Cora Lansquenet. Each receives an equal share of the estate. Cora claims that 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, can provide little information.
Miss Marple begins her investigations as a guest at the Gallop Hotel run by Hector Enderby, where the other two surviving heirs and Miss Milchrest are staying. When Inspector Craddock questions them and Rosamund Shane's spendthrift husband Michael, none of them can produce a satisfactory alibi for the time of Cora Lansquenet'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, to ask for money. 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 Mr 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 murderer, Miss Milchrest, makes one last attempt to silence her, but Miss Marple is ready. Milchrest's motive is revealed to be a seemingly worthless painting owned by Cora, which actually was very valuable.
Hector Enderby later proposes marriage to Miss Marple but she turns him down. When she declares her distaste for blood sports, Hector, an enthusiastic fox hunter, mutters to himself "That was a narrow escape!"
Christie is mentioned by name early in the film when Miss Marple speaks to the police inspector after the first death. Later in the film, Marple also uses the phrase, "murder most foul," a line from Hamlet which also became the title of the next movie in the series.
