Christian Gilbranson, Miss Marple's lawyer, persuades her to visit the baronial estate of his step-mother, Carrie Louise Serrocold, an old friend of Marple's. Carrie Louise's devoted husband Lewis has turned the manor house into a halfway house for young men with criminal records and has hopes of expanding the facilities. He confides to Miss Marple that he suspects that someone is slowly poisoning Carrie Louise with arsenic. One of the young charges, Edgar Lawson, claims to be Lewis' illegitimate child and wants his alleged father to acknowledge him. While they are arguing behind closed doors a gun is discharged, and the visiting Gilbranson is found murdered in another room. It is up to Miss Marple to aid crusty Inspector Curry discover the murderer's and poisoner's identities.