While traveling by rail, Miss Marple witnesses the strangling of a young woman in the carriage of an overtaking train. The local police can find no evidence to support her story, so she conducts her own investigation and, with the aid of her close friend Jim Stringer (Stringer Davis), comes to the conclusion that the body must have been thrown off the train near the grounds of Ackenthorpe Hall, which adjoins the railway line.
Wheedling her way into a job as housemaid there, Marple copes with her difficult employer, Luther Ackenthorpe (James Robertson Justice), and searches for the missing corpse. She eventually finds it concealed in a stable, much to the chagrin of Police Inspector Craddock (Bud Tingwell).
Miss Marple has Mr. Stringer uncover the details of Ackenthorpe's will: the family fortune will go to his long-suffering, attentive daughter Emma, sons Cedric, Harold and Albert, and grandson Alexander. (A fourth son, Edmund, was killed in the war.) Also, Ackenthorpe's physician, Dr. Quimper (Arthur Kennedy), and Emma are secretly in love. Gardener Hillman and part-time servant Mrs. Kidder (Joan Hickson) round out the establishment (and suspects).
Alexander finds the first clue, a musical compact which plays "Frère Jacques", near where the body must have landed. When Emma reveals that she recently received a letter from a French woman named Martine, who claimed that she had married Edmund shortly before he died (and is therefore an heir), the identity of the dead woman and the motive for the crime seems clear.
Arsenic in the curry duck prepared by Miss Marple herself sickens all who eat it, but only Albert succumbs. Then Harold dies by his own shotgun. The police are unsure if it was suicide by a remorseful murderer or the third victim.
Miss Marple, however, is not deceived, and sets a trap, using the compact as bait. Dr. Quimper is revealed to be the villain. The dead woman was not Martine at all, but his wife. Quimper feared that the compact, a gift to his wife, could be traced back to him. He intended to dispose of the other heirs and marry Emma. He administered a second, fatal dose of arsenic while supposedly attending to Albert.
In Christie's original story, elderly Elspeth McGillicuddy witnessed the murder, not her friend Miss Marple, who was introduced later. Also, in the original story, a young acquaintance of Marple's is sent to pose as a house-keeper at the suspect location, not Marple herself. As with most of her portrayals of Miss Marple, Rutherford's interpretation was quite different from Christie's languid, passive depiction. The tone of the novel was also changed somewhat; instead of Christie's trademark suspense and underlying darkness, the film relied heavily on light, even whimsical comedy of manners.
The name of the manor house where Marple conducts her inquiries was called Rutherford Hall in the novel, and was changed to Ackenthorpe Hall in the film to avoid comparison with the leading actress's surname. Crackenthorpe, the family name in the novel, was shortened to Ackenthorpe.
Despite Christie's dislike of this adaptation, Murder, She Said received a generally positive response from critics, and maintains an 83% approval rating on Rotten Tomatoes. Almar Halfidason, a critic for the BBC film website, awarded the picture four stars out of a possible five, calling it "delightfully dotty" and "fun".
The film made a profit of $342,000.