When the ship that is carrying Leslie Calvin and her wealthy parents from Batavia to America sinks, Leslie, one of only four survivors, is haunted by the death of her parents. Just before she is to be released from the New Orleans hospital in which she is recuperating, Leslie writes a letter to her only living relative, her mother's sister, Emily Lamont, whom she has never met. Emily writes back from Belleville, Louisiana, explaining that she and her husband Norbert are residing at the ancestral plantation there and inviting Leslie to stay with them. Leslie travels to Belleville, but when no one appears to meet her at the train station, the neurotic Leslie faints from the heat. The town physician, George Grover, is summoned to treat Leslie and convinces her to accompany him to his office. There, Leslie confides her fears about being alone and her recurring nightmares about her rescue. Feeling compassion for the distraught Leslie, George offers to drive her to the plantation. There, they are met by the overbearing Mr. Sydney, who introduces himself as a guest of the Lamonts, and Leslie's eccentric aunt Emily, who claims that she never received Leslie's telegram notifying them of her arrival. Before departing, George cautions Sydney that Leslie is emotionally unstable and needs to forget her traumatic ordeal. After Emily escorts Leslie to her room, Sydney extracts her telegram from his coat pocket and tosses it in the wastebasket. At dinner that night, Sydney urges Leslie to relate the tale of her tragic voyage, sending her running from the table, hysterical. The next morning, Sydney and Cleeve, the overseer, take Leslie on a tour of the plantation and force her to tread perilously on a ledge along the bayou. As Cleeve is about to coerce the terrified Leslie into joining him for a boat ride, George appears and invites her to join him on his rounds. At a bayou shack, George introduces Leslie to the Boudreaux family, who ask them to lunch. Leslie's spirits are uplifted, until she attends a movie with the Lamonts and Sydney that night and views a newsreel depicting the sinking of a ship by a German submarine. The next day, as Leslie suns herself in the garden, Pearson Jackson appears to ask for her help. Pearson explains that he worked on the plantation for twelve years until the Lamonts arrived and Cleeve fired him. That night, George takes Leslie to a dance, and Leslie recalls that as a girl she would dance for her mother, who was unable to walk. When George kisses her and proposes, Leslie runs into the house, asserting that she can never see him again. Agitated, Leslie then confides to her aunt that she can never marry because she suffers hallucinations and belongs under the water with her mother and father. After going to bed, Leslie hears a voice calling her name and wanders outside, seeking its source. She is startled by Pearson, who has also heard the voice and warns her that spirits are pursuing her. Terrified, Leslie takes refuge in the house and phones George, who is out on a housecall. After leaving a message for George, Leslie questions Emily about Cleeve and Sydney and charges that one of them is trying to drive her insane. When Emily begins to reminisce about Leslie's mother's love of dancing, Leslie realizes that she is an impostor. The next day, Pearson warns Leslie that her aunt and uncle are impostors and arranges to meet her in the bayou that evening. When Leslie goes to the appointed meeting place, however, she finds Pearson's dead body. Leslie decides to catch the next train leaving Belleville, but is prevented from doing so by Emily, who summons her to her room. The next morning, George returns Leslie's phone call, and Leslie insists that he immediately come to the plantation. When he arrives, Leslie apprises him of Pearson's murder and claims that her aunt and uncle are impostors. Incredulous, George writes out a prescription for a tranquilizer. After sending Leslie to her room for a rest, George informs Sydney that he is certain that Leslie is suffering from delusions and will arrange for her to see a psychiatrist. In her room, Leslie, desolate, looks at the prescription and realizes that George has actually written a note, warning of her danger and promising to return with help. After George departs, Sydney addresses Norbert as Pinky and Emily as May. When May, who has been hired by Sydney to pose as Emily, objects to harming Leslie, Sydney reminds her that she and Pinky are already implicated in the Lamonts' murder. At the boathouse, meanwhile, Cleeve has taken George prisoner. After George tricks Cleeve into admitting that he murdered the Lamonts, he begins to taunt Cleeve about performing Sydney's dirty work. Soon after, Sydney appears with Leslie. After demonstrating how he used a phonograph to call out Leslie's name, Sydney explains that he engineered the diabolical plot to drive Leslie mad and acquire her inheritance. Sydney then orders Cleeve, George and Leslie into a motorboat, but when he commands Cleeve to kill George and Leslie while motoring into the bayou, Cleeve balks and the two men begin to argue. In the confusion, Leslie and George jump overboard and hide in the water lilies. When Leslie collapses with terror, George helps her ashore and Cleeve and Sydney follow. Cleeve charges into the swamp, but sinks into a bog of quicksand and drowns. George then calls to Sydney and offers to lead him out of the swamp in exchange for his gun. After Sydney discards his weapon, George picks it up and orders him back into the boat. When Leslie climbs in and follows George's instruction to start the engine, she realizes that she is recovered at last.