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At a villa on the Italian Riviera, Paul Decker meticulously executes a plan to murder his British wife Madge and make it look as if she committed suicide. After sedating Madge with a glass of drugged milk, Paul drapes her body across the couch in the sitting room, seals all the windows with tape and locks the door from the inside. Then, after donning a snorkel and diving mask, he turns on the gas lamps and climbs down a trap door hidden underneath a rug to await her demise. When Madge's lifeless body is found by the maid and gardener the next morning, the police, accompanied by Mr. Wilson, the British Consulate, are summoned to investigate. Madge's teen-aged daughter Candy also arrives, chaperoned by her companion, Jean Edwards, and dog Toto. Upon learning of her mother's untimely death, Candy accuses her stepfather of murder. When the police inspector questions Jean about Paul, who is allegedly in France writing a book, Jean tells him that Candy believes that Paul deliberately killed her father in a boating accident years earlier. After Wilson drives Candy and Jean to a hotel in town, Paul emerges from a tunnel beneath the trap door and sneaks out of the house. Later, the Inspector phones Jean to read her a letter from Paul that has just been delivered to the house. In the letter, Paul writes that he is on his way home, prompting Jean to hurry to the villa to break the news of Madge's death to him. At the villa, Jean comforts the "grieving" Paul. When Jean returns to the hotel, Candy insists that her mother would have left behind a farewell letter if she was contemplating suicide. After Madge's death is ruled a suicide, Candy goes to see the inspector and accuses Paul of killing her mother. When the inspector wonders how Paul could have survived in a room filled with gas, Candy vows to solve the mystery. Upon returning to the hotel, Candy glances out the window in her room and sees a poster of a skin diver. Later, when Jean tells Candy that Paul has suggested that she spend time with her aunt in America, Candy asks about the mechanics of using a passport to travel from country to country. When Jean explains that the passport is stamped each time its owner enters and leaves the country, Candy realizes that if Paul was truly in France at the time of the murder, his passport would bear a French stamp. Candy decides to investigate, and takes Toto to Paul's room to look for the passport. While Candy searches for the passport, Toto pulls a snorkel out of the wardrobe. Just as Candy is leafing through the pages of Paul's passport, Paul suddenly appears and shows her the stamp, proving that he was in France. After Candy leaves, Toto brings the snorkel to Paul, who then poisons the dog. While Toto returns to Candy's room to curl up in a corner and die, Candy tells Jean that her mother was terrified of the gas lamps in the house and begged Paul to install electricity, but he refused. When Candy finds Toto's lifeless body, she accuses Paul of killing him, too. The day before they are to leave Italy, Paul takes Jean and Candy to the beach. Seeing a man snorkeling, Candy asks Jean about how a snorkel works. When Jean explains that the device allows one to breathe underwater for extended periods of time, Candy mentions Paul's snorkel and deduces that he must have used it to stage her mother's suicide. After Candy swims out past the shoreline, Paul feigns concern for her safety and follows her. Just as Paul shoves the girl's head underwater, Jean arrives, and they pull the gasping Candy to shore. Upon reviving, Candy accuses Paul of trying to drown her, prompting Jean to slap the girl and declare that she must be mentally deranged. Back at the hotel, Paul tells Jean that he plans to leave that night to spare Candy any more anguish. Meanwhile, Candy goes to tell the inspector about the snorkel, and upon learning that it is his day off, leaves a message for him to call her. Later, Paul says goodbye to Jean and drives to a hotel on the French border. After checking in, Paul sneaks out with his snorkel and swims back to Italy. A ringing phone interrupts Candy's sleep, and when she answers it, Paul, impersonating an officer from the inspector's office, directs her to go to the villa. Intending to kill Candy in the same manner that he killed her mother, Paul seals the windows of the sitting room. When Candy arrives, Paul tells her that the inspector is on his way and explains that he found a letter from her mother. Offering to read the letter aloud, Paul invites Candy into the sitting room. Paul then recites Madge's "last words." The letter purportedly states that she decided to take her own life after being diagnosed with a terminal illness, and admonishes Paul and Candy to take care of each other. When Candy breaks into tears, Paul puts down the pages and offers the grieving girl a glass of drugged milk. After drinking the milk, Candy notices that the pages of the letter are blank and that the windows have been taped shut. Before she can escape, she passes out. Paul then drapes her body over the couch, removes the glass, locks the door, dons his snorkel and turns on the gas. Just then, he hears a car door slam, and throwing the trap door open, bolts into the tunnel. Running up the stairs, Wilson and Jean break down the door and open the windows. When Candy awakens, she declares that Paul tried to kill her and is still hiding in the room. Believing that Candy tried to commit suicide, Wilson placates her by agreeing to search the room on the condition that if Paul is not found, Candy will give up her delusions of murder. In his search, Wilson moves a heavy cabinet away from the wall and leaves it standing on top of the trap door, thus condemning Paul to an airless death. When Wilson's search proves fruitless, Candy agrees to depart with him and Jean. Running back to the room for one last look, Candy hears Paul calling for help, then turns around and leaves him to die. On the drive out of town, they pass the inspector's office and Candy asks Wilson to stop. Relenting, Candy enters the office and tells the inspector to go the villa and look underneath the sitting room floor.