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They do it with mirrors - Agatha Christie

Jane Marple is paying a visit to her old friend Ruth Van Rydock, a wealthy, oft-married socialite, whom she went to school together in Italy, along with Ruth's sister Carrie Lousie, a woman who has always been attracted to men who had their minds on noble causes. During the visit, Ruth voices her worries that there is something wrong at her sister's home at Stonygates, a Victorian mansion that Carrie Louise has outright ownership of, but cannot give any real reason for these worries, only that she fears that her sister is in some kind of danger. Ruth therefore asks Miss Marple to visit her and find out what is going on.

Travelling to Stonygates, Carrie Louise is delighted to have Miss Marple visit her. Jane learns that the mansion has been converted into a home for delinquent boys, all of whom are involved in theatrical productions and many other activities around the estate during the day, but at night they are confined to their own quarters. The central block of the mansion is reserved to Carrie Louise's family, who include Lewis Serrocold, her third husband, and the man who runs the reformation program at Stonygates, fully devoted to the idea of reforming juvenile delinquents and teaching them how to contribute to society; Mildred Strete, her daughter from her first marriage, and her only blood relative; Stephen and Alex Restarick, her stepsons from her second marriage (which ended in divorce), and frequent visitors to the estate; Gina, her adopted grand-daughter whose mother Pippa, Carrie Louise's adopted daughter, died after her birth; Walter Hudd, an American married to Gina, who recently returned to Stonygates; and Juliet Bellever (nicknamed Jolly), a long-time companion, caretaker, and friend of Carrie Louise, who is a permanent fixture at the mansion. Also frequently present in the mansion is Edgar Lawson, Lewis Serrocold's assistant, and an awkward young man whom the others dismiss as pompous, as well as being half-mad because of several occasions in which he confided to others that he is the illegitimate son of a great man, and claimed that powerful enemies are conspiring to keep him from his rightful position.

While at Stonygates, Miss Marple witnesses Mr. Serrocold receiving an unexpected visitor in the form of Christian Gulbrandsen, Carrie Louise's stepson from her first marriage, and a member of the Stonygates Board of Trustees; her first husband, Mr. Gulbrandsen, was a gret philanthropist, and left a fortune for her in trust. Everyone assumes he is there on business, but no is sure exactly why. After dinner, following his arrival, Christian retires to his guest room to type a letter, while Miss Marple and the others gather in the Great Hall. At that moment, a fuse blows out, and Walter goes off to repair it. Suddenly, Edgar Lawson bursts into the darkened room, screaming that Lewis Serrocold is his real father, forcing him into his study and locking the door behind him. Everyone listens intently as Edgar screams accusations at Mr. Serrocold, before hearing multiple gunshots. When the door to the study finally opens, everyone is surprised but relieved to see that Mr. Serrocold is alive and well, while Edgar is in tears, and that there are several bullet holes in the walls. However, when Miss Bellever goes to check on Christian, she finds him dead, having been shot at his typewriter, with the letter he had been writing having mysteriously disappeared.

When the police arrive at Stonygates, Lewis reveals to the police that he had taken the letter, intent on keeping his wife from learning of its contents. He explains that both he and Christian had been both concerned that Carrie Louise's recent poor health was due to deliberate poisoning. At that point, Alex Restarick arrives, whereupon police question him over his sudden arrival, and find that there is an unaccounted period of time between his arrival in his car and his appearance in the Great Hall. As the investigation into the murder continues, Alex makes remarks about stage scenery that lead to Miss Marple reflecting on all kinds of stage illusion, such as conjurers who perform magic by using mirrors and stage sets and assistants who are in on the trick. Soon afterwards, Alex is murdered, along with a boy who claimed to have seen something on the night of the murder. When Miss Marple considers Alex's remarks in more detail, she realises how the murder of Christian was committed, why Alex was murdered, and who the culprit is.

It transpires that Lewis Serrocold was the murderer. Christian had discovered that he was embezzling from the Gulbrandsen Trust, thus he had to be silenced before this revelation became public. To do this, Lewis involved Lawson, who was in fact his illegitimate and unacknowledged son, into staging a scene for everyone in the Great Hall, using illusion and misdirection. Once in his study, with everyone focused on the study door, Lawson used his acting talent and different voices to continue both sides of their loud argument by himself, while Serrocold left via a terrace outside the study, re-entered the house unseen, and killed Christian. The attempted poisoning of his wife was merely a red herring, an explanation Serrocold hastily concocted to explain Christian's sudden arrival. Alex's murder was mainly because he had worked out what happened "behind the scenes" and thus could have easily exposed Serrocold's crime.

When confronted by the police, Edgar Lawson panics and flees the house, jumping into an old boat in an attempt to cross a lake on the property. The boat is rotted though, and as it begins to sink, Lewis Serrocold jumps into the lake to rescue his son. Both men are caught in the reeds that line the lake, and drown before police are able to rescue them, bringing an end to the case.