A mad scientist turns a gorilla into a beautiful young woman in this well-made Universal potboiler, the first of three films featuring Paula Dupree, the Ape Woman. John Carradine stars as Dr. Sigmund Walters, whose Crestview Sanitarium witnesses strange and unsettling experiments. The doctor's newest scheme concerns Cheena (Ray "Crash" Corrigan), a female gorilla that he has stolen from the Whipple Circus. Injecting the ape with sex hormones obtained from Dorothy Colman (Martha Vickers), the evil medico attempts to turn the animal into a semi-human creature. When Dr. Walter's longtime nurse, Miss Strand (Fay Helm), objects to this blasphemy, she is summarily murdered and her brain transplanted into the ape woman's skull. The result is named Paula Dupree (Acquanetta), a beautiful but mute creature. At the circus, Paula rescues lion tamer Fred Mason (Milburn Stone) from an attacking animal and a grateful Fred makes her his assistant. The team is highly successful but a lovesick Paula becomes jealous of Fred's girlfriend, Beth Colman (Evelyn Ankers), a condition that turns her into a half-ape, half-woman. Failing to kill Beth, Paula returns to the sanitarium, where Dorothy is being prepared for more experiments. The girl is rescued in the nick of time and an enraged Paula, now completely returned to simian form, kills Walters. Escaping, the ape once again saves Fred's life before being put down by an arriving police officer.