The formidable Lady Tressilian reluctantly hosts a house party at her Devon estate, where loyal companion Mary and old friend Treves help their wheelchair bound host entertain a glittering array of guests. Eyebrows are raised when the dashing Wimbledon tennis star Neville Strange arrives with his attractive but high maintenance new wife Kay, even though his elegant first wife Audrey is also attending. Miss Marple, an old school friend of Lady Tressilian, is intrigued when the eccentric Treves tells a story about a child murderer who was never convicted. The next day, Treves is found dead.
Sexual tensions and marital mishaps play out as Miss Marple observes the guests as they holiday by the sea. Whilst trying to reveal the killer's identity within this tightly wound group of people reveals, Mallard finds betrayal and revenge at the heart of it all.