A mad scientist transplants human glands into a gorilla, turning the ape into a beautiful young woman (Acquanetta). However, a severe emotional jolt soon reverts her back into her primitive self with disastrous consequences.

Animal trainer Fred Mason is warmly greeted by his fiancée, Beth Colman, upon his return from a two-year expedition in Africa. Along with an assortment of wild animals, Fred brings with him a female ape he has named Cheela. While the animals are being unloaded from the ship, a tiger escapes, but Fred manages to save the day by corraling the animal into a ticket booth. During the ride into town, Beth tells Fred about the treatment her younger sister Dorothy is receiving from the noted endocrinologist, Dr. Sigmund Walters at the Crestview Sanitarium. They then arrive at the winter home of the Whipple Circus, where Fred meets Walters and the circus' owner, John Whipple. Whipple and Walters inspect the new lions and tigers Fred has brought back from his safari, including Nero, a male lion notorious for killing four natives. Walters takes an immediate interest in Cheela, and when Whipple tells him that the ape is not for sale, he abducts the animal with the help of Gruen, a recently fired circus worker. After allowing the ape to kill Gruen, Walters injects Cheela with some of Dorothy's sex hormones, transforming the ape into a beautiful woman. When Walters' nurse of thirteen years, Miss Strand, objects that such experiments might kill Dorothy, the physician kills her and transplants her brain into the apewoman's head. With his experiment a success, Walters renames his creation Paula Dupree and begins training her for a new life. Meanwhile, Whipple agrees to hire Fred as the circus' new lion tamer after he fails to sign renowned animal traine Clyde Beatty for the position. As Fred practices his act, Walters arrives with Paula, and she saves Fred's life when he is knocked unconscious in the lion's cage. Paula is quickly hired as Fred's assistant, and their act becomes a great success. Paula's jealousy of Beth, however, soon causes her to excrete excessive amounts of animal secretions, turning her into a half-woman, half-ape. She climbs into Beth's bedroom in an attempt to kill her rival, but murders another woman instead. Paula then returns to Walters' laboratory, where the doctor lectures the animal, stating that he must now graft more sex glands and transplant another brain into her body to return her to human form. Meanwhile, Beth suggests that Paula and Cheela may be one and the same, an idea that is rejected as ludicrous by Fred. Beth then receives a phone call from Dorothy, in which her younger sister begs to be removed from Walters' care. Beth arrives at the sanitarium just as Walters is about to begin his latest operations on Dorothy and Paula, who has now fully returned to ape form. Learning of the ape's hatred of Walters, Beth releases the animal, which then kills the mad surgeon. Meanwhile, in the midst of an electrical storm, Fred performs his act, which, against Whipple's orders, now includes Nero. Frightened by the lightning, the lions and tigers become uncontrollable, and Nero attacks Fred, but the animal trainer is rescued by Cheela, who carries him out of the lion's cage and to safety. The heroic ape, however, is killed in the end by a policeman, who mistakes the true intentions of the animal.