At a coroner's inquest, Dr. Carl Fletcher is questioned about the mysterious death of Paula Dupree. The physician confesses to her murder, but is initially reluctant to explain the circumstances of Paula's death. Carl finally tells of going to the circus the night lion tamer Fred Mason was saved from death by an ape named Cheela, who was then mistakenly shot by a policeman. Though the ape is thought to be dead, Carl takes the animal back to his laboratory and nurses it back to health. Later, Fred visits Carl and tells him that natives back in Africa thought Cheela was a human being who had been transformed into an ape. Fred also tells Carl about Dr. Sigmund Walters' interest in Cheela, and how after Cheela's disappearance, Walters brought Paula to the circus, where she became Fred's assistant. Fred and his wife Beth then take the stand, and their testimony also suggests that Cheela and Paula were one in the same. Carl returns to the stand, and talks about buying Walters' sanitarium, where, on the night that Cheela escaped, Carl's dimwitted assistant Willie finds the beautiful, but mute, Paula roaming the grounds. Diagnosed by Carl as suffering from shock, Paula begins to speak when she sees Bob Whitney, the boyfriend of Carl's daughter Joan. Paula becomes insanely jealous when Bob and Joan announce their engagement, and when the couple later go canoeing on a nearby lake, the apewoman tips over their boat and attempts to drown Joan. The next morning, Willie is discovered missing and he is assumed to be the one who overturned the canoe. Later, Paula goes to Bob and accuses Carl of mistreating her, only to have Joan walk in on them and think the worst. Paula then attacks Carl, and when the physician defends himself, Bob enters the room and comes to believe Paula's accusations. Later, Carl has Paula's fingerprints compared to those found on the broken lock near some dead animals on the grounds of the sanitarium, and they are an exact match except for their size. Meanwhile, Bob has Paula examined by another physician, who states that the disturbed woman should be returned to Carl's care. Willie's crushed body is then discovered by the lake, and Carl comes to realize that Cheela and Paula are one in the same. Returning to the sanitarium, Bob tells Paula that he is still in love with Joan, but before the apewoman can kill Joan, Carl arrives on the scene. The two struggle, but Carl manages to give Paula a tranquilizer. In his haste, however, the physician injects a fatal overdose of the sedative. At the end of Carl's testimony, the coroner orders the court to examine Paula's body, and they are shocked to discover that the dead woman's body has transformed into that of an apewoman. Carl is then cleared of all charges, and leaves the morgue arm and arm with Bob and Joan.