Ruth van Rydock is worried about her sister's health and well-being and asks their mutual friend Jane Marple to visit her for a awhile. Her sister, Carrie-Louise Serrocold, is married to Lewis Serrocold and he has turned their estate into an institute of sorts for young offenders. Miss Marple is more than happy to visit and finds an odd assortment of characters on the premises. When Christian Gulbranson is found shot while sitting at the typewriter in his room, Miss Marple has a murder to solve. Gulbranson, a trustee of the institute, had only arrived that day on urgent business and Lewis Serrocold found a partly completed letter in his typewriter saying that someone was poisoning Carrie-Louise. Assisting Inspector Slack, with whom she had previously worked, she soon knows who committed the crime but is not quite sure how they did it.

Wealthy business man Rex Fortescue collapses when he reaches his office. It is determined that he was poisoned. No one seems too upset, as everyone agrees that Rex was a tyrant. Soon however Rex's young wife also turns up poisoned. The maid, Gladyes, whom Miss Marple trained, writes to Miss Marple asking for help. Miss Marple arrives too late, as Gladyses' body is found strangled and strung up on the clothes line. Now it's personal, and Miss Marple is determined to see the murderer punished.

A friend of Miss Marple's sees a woman being strangled in a passing train. When police cannot find a body and doubt the story, Miss Marple enlists professional housekeeper, Lucy Eyelesbarrow, to go undercover.

At a reception for a fading film star making a screen comeback, a gushing, pushy fan is poisoned by a drink apparently meant for the actress.