Under a sodium pentothal session administered by two psychiatrists, Joyce Webster begins to unlock memories of her past as nurse Jane Marvin. The memories unfold in vivid flashbacks: Paul Webster and Joyce marry, and on their honeymoon train he abruptly disappears after receiving a telegram. As months pass, Joyce hires private detectives and scours the countryside for him, ultimately tracing a lead to the address of the Cypresses Plantation that Paul wrote down on his college enrollment forms.
Joyce travels to the desolate town of Bayou Landing, Louisiana, where she encounters Lavinia Hawthorne, the plantation’s stern mistress. Lavinia is wary and calls Joyce a liar, even trying to have her escorted away. Yet when Joyce explains that she has missed the last train back to town, Lavinia reluctantly allows her to stay the night on the condition that she remain in her room. That night, Joyce hears a piano playing and slips downstairs to investigate. There, she glimpses a shadowy figure in a trench coat at the piano—the man she once knew as Paul Webster. When Joyce enters, Paul bolts, and he later urges Lavinia to make Joyce leave.
The next morning a local doctor, Dr. Mark Sinclair, arrives to question Joyce. Sensing that everyone is withholding information, Joyce refuses to depart. When she presses Lavinia for the truth about Paul, Lavinia breaks down and confesses that Paul is her son, a revelation that deepens the mystery rather than resolves it. Joyce’s vigilance continues as she follows Paul into the swamps, where she faces dangers from alligators and a giant snake. She is rescued by Manon, the plantation handyman, who carries her back to his shack. There, he assaults her, and Paul bursts in to intervene, striking Manon and then taking Joyce back to the house. Manon, enraged by the sight of his rival, vows to kill Paul.
Back at the plantation, Paul presses Dr. Mark Sinclair to administer an untested cobalt treatment in an effort to reverse his condition. Mark agrees and moves Joyce to a clinic where the experiment is set up. The next morning, Mark explains his work: years earlier, after Paul was badly mangled in a plane crash, he experimented with a serum derived from reptilian hormones that could regenerate limbs. The treatment initially appears successful, but the patients begin to take on reptilian traits. Learning of Paul’s plan to undergo cobalt treatment, Joyce insists on being present.
Paul confronts Joyce at the clinic and, though he is ashamed, she reassures him of her love. The cobalt procedure begins, but chaos erupts when Manon bursts into the laboratory and destroys the control panel. The machine releases powerful rays that morph Paul into a bipedal reptilian monster with an alligator-like head. In the struggle, Manon is electrocuted by tangled cords, and Paul’s voice is replaced by a snarling reptilian sound as he fights to communicate. Hearing Joyce’s screams, he flees into the swamps. The cobalt machine short-circuits, self-destructs, and the lab is destroyed. Paul stumbles into quicksand and gradually sinks from sight, seemingly meeting his end.
In the present, the psychiatrists review the tapes of Joyce’s ordeal. They conclude that her amnesia allowed her to suppress the horror and return to a normal life, and they decide not to reveal her past to her. The film closes on a note of quiet ambiguity, with Joyce continuing to live in the shadow of memories she has chosen to leave behind.
