The ordered tranquility of life in St Mary Mead is shattered by a mysterious discovery. Miss Marple's investigations lead her to a seaside hotel where some rather unusual people are staying.

Miss Marple is convinced that the death of the schoolgirl Pamela Reeve holds the key to the mystery of the double murder. There are a number of suspects, but the police finally arrest Basil Blake. Miss Marple, however, draws an entirely different conclusion. After a train journey to London, she comes back with evidence that allows the police to stage a trap for the real killer.