Sensing danger surrounding her sister Carrie-Louise, Ruth Van Rydock enlists the help of old friend Miss Marple to pay a visit. Carrie-Louise and her third husband, Lewis Serrocold, run a correction facility for young men on their estate, where they also house a rag-tag assortment of family and step family.
As Carrie-Louise's family prepare a musical revue for the inmates of the facility, Miss Marple senses all is not right beneath the veneer. Lewis muddles through a full dress rehearsal in front of the family, but is interrupted by his secretary, the troubled young Edgar Lawson, who is wielding a gun. Edgar is wrestled to the ground by a surprise visitor, Carrie-Louise's second husband, the theater impresario Johnny Restarick. While the drama unfolds, the maid makes a shocking announcement — Carrie-Louise's stepson from her first marriage, Christian Gulbrandsen, who forms part of the trust that maintains the correctional facility, has been murdered in the study.
Examining the note Gulbrandsen was typing before being stabbed, detectives Inspector Curry and Sergeant Lake ascertain that he was trying to warn Carrie-Louise of an attempt to poison her. Marple pierces Inspector Curry's patronizing preconception of her in order to help discover the source of the threat. There appears to be no end of hidden resentments and agendas under Carrie-Louise's roof. Miss Marple realizes that the only way she will discover the root of the danger is if she unravels the conjuring trick played on the night of the murder.
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Warning: Contains significant plot spoilers
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Warning: Contains significant plot spoilers
During tea with her old school chum Ruth Van Rydock, Miss Marple learns that Ruth's eccentric sister, Carrie Louise, has lost her personal papers in a suspicious fire and that someone may wish to do her harm. Miss Marple agrees to investigate.
Arriving at Carrie Louise's estate, Miss Marple finds that it doubles as a home for delinquent young men, thanks to the enlightened social conscience of her (third) husband, Lewis Serrocold, then organizing an amateur play to entertain the inmates. His personal assistant is a troubled youth named Edgar Lawson, who is convinced he is Winston Churchill's son.
The household includes Carrie Louise's daughters from her first marriage: Mildred, who is quiet and embittered, and Mildred's more glamorous adopted sister, Gina, who is married to a taciturn former GI from Wyoming, Wally Hudd.
Also resident is Carrie Louise's stepson from her second marriage, Stephen Restarick, a theater maven like his producer father, Johnny, and a shameless flirt with Gina during rehearsals for the house show.
Appearing unexpectedly is Carrie Louise's stepson from her first marriage, Christian Guldbransen, a silver-haired businessman who is much older than his step-siblings, and who arranges an urgent private meeting with Lewis.
In the evening, all except Christian, who is upstairs typing a letter, assemble for a run-through of the show. Suddenly, the lights go out and Edgar appears with a gun, threatening Lewis; the gun and threats are real.
Everyone reacts differently. Wally disappears to restore power. Lewis flees outside. Stephen plays the piano as if nothing is wrong. Others scramble in the dark. The surprise arrival of Johnny from London saves the day as he disarms Edgar.
However, in the interval Christian has been murdered in his room, leaving a partially written letter warning of a poison plot against Carrie Louise. It later emerges that Gina is the daughter of a woman hanged for a notorious string of poison killings, casting suspicion on her.
Inspector Curry and Sergeant Lake take charge of the case, discovering a very useful colleague in Miss Marple. The three sleuths actually witnesses the next homicide, when Johnny is killed by falling scenery during the premiere of the play.
Taking the theater theme of the murders as a clue, Marple realizes that she and the police have been deceived by a showman's sleight-of-hand. Lewis, she announces, is the mastermind.
It seems he engineered the power failure at the rehearsal, coached Edgar, who turns out to be his son, to play a deranged assailant, and then disappeared during the commotion to run upstairs and stab Christian in the back, using Wally's army knife.
The motive? Lewis was trying to hide fraudulent dealing in the family trust. He had already torched Carrie Louise's papers in the suspicious fire. Then Christian discovered the fraud and was writing an account to Carrie Louise, Lewis killed him and made it appear that the letter concerned a poisoning scheme-a red herring designed to implicate Gina. After Johnny became suspicious of the poisoning story, he was dispatched by Edgar.
Regrettably, Lewis and Edgar drown in an accident following the revelation of their misdeeds.