Marple is invited to London to see her old friend Ruth. Miss Marple was a sort of traveling companion to Ruth and her sister, Carrie Louise. Ruth is worried about Carrie Louise. She doesn't know why, though. Ruth asks Miss Marple to come with her to her sister's estate, Stonygates. Stonygates is a huge manor house, and some of the buildings and the grounds are used by Carrie Louises' current husband, Lewis, as a reformatory for juvenile criminals. Miss Marple meets the people who live there - Carrie Louise's grand-daughter Gina and her malcontent American husband Walter; Stephen and Alex Restarick, Carrie Louise's stepsons by her first marriage; Edgar Lawson, Lewis' mentally unstable secretary; and Carrie Louise's daughter, Mildred, who is jealous of Gina. Soon, Carrie Louise's stepson by her second marriage shows up. He is Christian Guldbrandsen, and he runs the trust which contributes a fraction of the funds used to run Stonygates. He and Lewis talk in private. Later that night, Edgar begins waving a gun around and ranting that Lewis is his father. Lewis takes him to his office where the argument becomes even more heated. After a few minutes, two shots are heard, and then hysterical crying. The door opens to reveal Lewis, unhurt. Edgar missed him. However, the body of Christian Guldbrandsen is discovered, shot. Miss Marple sets out to find the murderer before it is too late for her good friend Carrie Louise.