At the train station in Tarnmoor, England, mystery writer Janet Frobisher calls her lover, Larry Stevens, who is engaged to her secretary, Chris Dale, and begs him to visit later. She is spotted leaving the station by her inquisitive neighbor, veterinarian Dr. Henderson, who questions why she is using a pay phone. He drives her home, where she is shocked to find a stranger in her living room. He introduces himself as George Bates and reveals that he and Janet’s long-lost husband, Preston, recently robbed a bank together. After shooting a policeman and planting the gun on George, Preston ran off with all the money, and George now desperately insists on speaking with him. Janet calmly tries to deter George, stating that she has not seen Preston in years, but George points to Preston’s hat and slippers, forcing her to admit that she killed Preston hours earlier. Shocked, George checks the lifeless body in the study, while Janet explains the events that led up to his death: She left Preston, an abusive husband, three years earlier and assumed her pen name of Frobisher. Over the years, she pretended her husband was in Malaya but he remained in England, continually blackmailing her. When he showed up that night vowing never to divorce her, she poisoned him with a compound used to treat her beloved horse, Fury. George realizes that the police will assume he has killed Preston and so demands that Janet pretend he is her husband, whom no one in town has met. He kisses her, but she slaps him. Just then, Henderson arrives and, while George hides, asks Janet to return the horse compound. Janet explains that she used it all, arousing Henderson’s suspicions, and after he spots Preston’s hat on the couch, insists on meeting him. George introduces himself as George Preston and, after Henderson leaves, informs Janet that now she will have to continue the charade. They drag Preston’s body to the deep lake outside, but when Larry arrives with Chris, George submerges the body by himself while Janet welcomes her guests, only one of whom she is pleased to see. The next morning, George fends off local Mr. Bigley, who has been sent by Henderson to request that he lecture about his travels at the next town meeting. Meanwhile, Janet and Larry ride horses on the moor, where Larry declares that although he cannot resist his attraction to Janet, he loves only Chris. When Janet returns to the house, George rails at her for leaving him alone and declares that he is staying indefinitely. Janet counters by lying that Preston was merely passed out, not dead, when George drowned him. A distressed George stays in his room until morning, when he runs into housekeeper Mrs. Bunting, who is puzzled that “Mr. Preston” looks so different from the photograph of him she has seen in a drawer. Later, Chris informs George that Janet and Larry are out riding again. Suspicious, George advises Chris to watch her fiancé carefully, then rides onto the moor. Before he can find Janet, however, Henderson drives by and, handing him the London newspaper, announces that a man looking exactly like George is being hunted for robbery. George rebuffs Henderson and returns to the house, but Henderson follows and there declares that, as an “amateur psychologist,” he has noted that George seems to be running away from something. Henderson leaves just as Janet and Larry return, and while George harangues Janet about her morning activities upstairs, Chris does likewise to Larry downstairs. George and Janet struggle over her gun, but when he grabs it, she gloats that now it has his fingerprints on it. She offers him Preston’s money and passport if he will leave, but after he points out that they are both better off sticking together, she reluctantly agrees. During a rainstorm the next day, an inebriated George raves at Larry that he should marry Chris quickly before he loses her, and warns that Janet is “poison ivy.” When Janet enters the room, George announces he is taking Fury for a ride, and a fuming Janet fails to inform him how difficult the high-spirited Fury is to control. Later, Chris appeals to Janet not to steal the only man she loves, but Janet coldly states that she wants him. Chris departs for the train station, after which Janet begs Larry not to leave her alone with George. Just then, George returns with the news that Fury broke his leg and had to be shot, causing Janet to rage that Fury was the only being she loved. Disgusted, Larry rushes after Chris, at the same time that Henderson arrives. The vet informs Janet in private that Fury’s leg was not broken, then announces that he is leaving his Jeep outside, as its brakes cannot be trusted in the rain. Inspired, Janet tells George that she wants them to be together, and asks him to bring Chris back so Larry can make up with his fiancée. George kisses Janet passionately and races out into the rain, where the Jeep swerves dangerously on the rain-slicked streets. Meanwhile, Larry returns from the train station, having arrived too late to stop Chris, but announces his intention to win her back. Janet throws him out, laughing bitterly that she has gone to so much trouble for nothing. Soon after, George staggers in, injured but alive, and Janet nurses him. The next day, Henderson prods Mrs. Bunting for inside information about George, and as she reveals that she has a photograph of Preston, George overhears their conversation and confronts Henderson. Henderson then reveals that Janet knew the Jeep’s brakes were faulty. After George storms out, Henderson tells Janet that he has spotted a body in the lake and called the police to drag it, then leaves. Desperate, Janet pours the horse compound into George’s flask and almost drinks it, but cannot, and instead pleads with George to stay with her. He agrees only to share a drink, but then, sure she will poison it and blame Preston’s murder on him, unknowingly drinks from his own poisoned flask. Janet laughs and watches him die. Just then, Henderson bursts in, and Janet announces that George killed Preston, forced her to pretend that he was her husband and then committed suicide. When Henderson reveals that Preston knocked on his door by accident days earlier, however, Janet faints in surprise, and Henderson holds the flask to her lips to revive her. She wakes, and when she sees the flask, her bitter laugh rings through the house.