After she is administered the drug pentathol by psychiatrists Dr. Erik Lorimer and Dr. Wayne McGregor, nurse Jane Marvin recalls a series events from her forgotten past when she was known as Joyce Webster: Joyce has just married a young man named Paul Webster. Aboard their honeymoon train, Paul receives a telegram and, in a panic, immediately leaves the train to make a phone call. When the train pulls out, Paul is missing, having vanished without a word to Joyce. Throughout the following months, Joyce employs private detectives and conducts her own search for her husband to no avail, until one day, she discovers the address of the Cypresses Plantation that Paul entered on his college enrollment forms. Joyce takes the next train to the whistle stop town of Bayou Landing in the heart of Louisiana swamp country. While sitting forlornly at the rail station, she meets Mannon, a handyman at the Cypresses, and asks him to drive her there. As they proceed deeper in the swamps, Joyce is horrified when Mannon tries to run over an alligator and then exhibits the hook where a gator bit off his hand. At the plantation, Joyce introduces herself to Lavinia Hawthorne, the Cypresses' stern mistress. When Joyce suggests that Paul once lived at the plantation, Lavinia bristles and calls her a liar. Joyce has missed the last train back to town, so Lavinia reluctantly invites her to stay the night under the proviso that she not leave her room. That night, Joyce is disquieted by the sound of gunshots, and when she tries to open the door to her room, she discovers it is locked. When the maid Lou Ann delivers Joyce's dinner tray, she warns that the house is deeply troubled and advises her to leave as soon as possible. Later, Lavinia notifies Mark Sinclair, a doctor who operates a clinic on the plantation, that Paul's wife is there. At the clinic, Mark administers an injection to an agitated patient who is swathed in bandages. Soon after, Lavinia arrives to confer about how to deal with Joyce. At the house, meanwhile, Joyce hears the strains of a piano and slips out of her room to investigate. As she descends the stairs, she sees a man in a robe, his face in shadows, seated at the piano and fails to recognize the shadowy figure as Paul. When Joyce enters the room, Paul flees, leaving behind a trail of muddy footprints. Paul, his face terribly disfigured, stops Lavinia's car and in a distorted voice, insists that Joyce leave as soon as possible. The next morning, Mark comes to the house to question Joyce, and sensing that he is withholding information about Paul, she refuses to leave. When Joyce demands that Lavinia tell her what she did to Paul, the older woman breaks down and confesses that Paul is her son. That night, as a storm rages, Paul, thinking that Joyce has gone, returns to the house. When Joyce sees him, he runs away and she follows him into the swamps. After a giant snake blocks her path, Joyce screams, and Mannon appears and carries her to his shack, where he assaults her. When Joyce tries to resist, Mannon slaps her unconscious, and Paul then bursts in, knocks Mannon down and takes Joyce back to house. After his mother insists that Joyce be told the truth, Paul presses Mark to give him an untested cobalt treatment in hopes of curing his condition. Mark reluctantly agrees to give him the treatment the following evening after Joyce has been informed of the situation. The next morning, Mark summons Joyce to his lab and tells her about his experiments with reptilian hormones that are capable of regenerating limbs. He continues that after Paul was horribly mangled in a plane crash, Mark administered the serum to him and several other accident victims. The treatment appeared to be a great success, until his patients began to turn into reptiles. Mark explains that after Paul received the telegram notifying him that his tests were positive, he hurriedly left the train and came home in hopes of reversing his condition. When Joyce learns of Paul's scheduled radical cobalt treatment, she insists on being present. That night, Paul encounters Joyce at the clinic and turns away from her in shame. After Joyce clasps his hand and reassures him of her love, Lavinia apologizes to her for her brusqueness. As Paul climbs onto the table and Mark aims the ray at him, Mannon bursts into the lab and destroys the control panel, shooting powerful rays at Paul that transform him into a giant alligator. Hearing his wife and mother scream in horror, Paul flees into the swamps and peering into the water, sees his reflection. Joyce scrambles after him, but after wrestling an alligator, Paul stumbles into quicksand and slowly sinks out of sight to the sound of Joyce's shrieks. Back in the present, the psychiatrists review the tapes of Joyce's ordeal and, concluding that her amnesia has allowed her to suppress the horror and resume a normal life, they decide not to tell her about her life as Joyce Webster.