Miss Marple, a shrewd and observant elderly woman, is traveling on a train when she sees a murder being committed on the tracks beside her. However, the police are skeptical and cannot find any evidence to support her claim. Unwilling to let a murderer go free, Miss Marple takes matters into her own hands and embarks on an investigation to uncover the truth. Her investigation eventually leads her to Ackenthorpe Hall, a grand estate where she takes a job as a housekeeper to gain access to the suspects. Her cover is quickly blown and she finds herself in danger as she gets closer to the truth. Along the way, she is aided by her friend Mr. Stringer, a local librarian, and together they unravel the mystery of the murder. The film is a delightful blend of humor, intrigue, and suspense, with Margaret Rutherford's portrayal of Miss Marple endearing her to audiences.