The film begins with animal trainer Fred Mason (Milburn Stone) returning from his latest safari with a horde of animals for his employer John Whipple (Lloyd Corrigan), owner of the Whipple Circus. Among them is Cheela (Ray Corrigan), a gorilla with remarkably human characteristics. Mason relates that she is the most affectionate jungle animal he has ever encountered.
Mason’s fiancée Beth Colman (Evelyn Ankers) is present at the dock for his return. She tells him of the recent health problems encountered by her sister Dorothy (Martha MacVicar). In a flashback sequence, Beth tells of taking her sibling to see Dr. Sigmund Walters (John Carradine), an endocrinologist of some standing. Dorothy is staying at Walters’ Crestview Sanatorium for treatment.
Fred and Beth arrive at the winter quarters, and Dr. Walters pays a visit. He is extremely interested in Cheela, and inquires about purchasing her. Whipple tells him that she is not for sale. Upon returning to his lab, Walters finds that his latest experiment has resulted in the lab animal’s death. He becomes convinced he needs larger animals that possess the “will to live.”
Walters enlists the aid of a disgruntled former circus employee to steal Cheela. After the ape is loaded onto his truck, the scientist callously pushes the man into the gorilla’s grasp and stolidly watches as the beast wrings his neck.
Back at his lab, Walters and his assistant Miss Strand (Fay Helm) transplant glandular material from Dorothy into Cheela. There were mentionings by Miss Strand that Walters has previously grafted the glands of different animals like placing a guinea pig's glands into a rabbit and a frog's glands into a mouse. To the horror of the nurse, the ape transforms into human form (Acquanetta). Telling the doctor that she cannot allow him to continue, Miss Strand informs him that at best he will have “a human form, with animal instincts.” Dr. Walters reaches the conclusion that he will need to place a human brain into his creation to successfully complete his experiment. He sacrifices Miss Strand for this purpose.
The brain transplant is a success, and the result is a sultry and exotic young woman who remembers nothing of her previous existence. Walters names her Paula Dupree, and takes his creation to the winter quarters for her first public outing. While watching Mason practice his animal act, an accident occurs. Paula rushes into the cage and saves him from the ferocious felines, who display an unnatural fear of her and retreat from her presence. Mason is dumbfounded and offers the girl a job in his act.
After the final dress rehearsal, Paula becomes jealous of Mason’s fiancée. She goes to her dressing room and while having a tantrum, begins converting to animal form. Later that night, she climbs through Beth’s window planning to kill her, but attacks and brutally murders another woman instead.
The beast returns to Walters, and the doctor realizes that another operation is necessary to return her to human form. He can continue to use Dorothy for the glandular material, but will need yet another subject to replace Paula’s damaged cerebrum.
Beth receives a frantic telephone call from her sister who expresses her fear of Dr. Walters and the forthcoming operation. Arriving at the Sanatorium to aid her sister, Beth is pegged by the good doctor as the next brain donor for Cheela. However, she proves resourceful in a pinch, releasing the ape from its cage. Cheela does Walters in and departs the lab, leaving Beth and Dorothy unharmed.
Performing his animal act solo, Mason finds himself trapped inside the cage with his unruly subjects. A powerful storm interrupts the performance and the beasts attack the trainer. Cheela comes to his rescue once again and carries him to safety. Unfortunately, a nearby police officer mistakes her intentions and kills Cheela.Evelyn Ankers - Beth Colman
John Carradine - Dr. Sigmund Walters
Milburn Stone - Fred Mason
Lloyd Corrigan - John Whipple
Acquanetta - Paula Dupree
Martha Vickers - Dorothy Colman (as Martha MacVicar)
Fay Helm - Nurse Strand
Vince Barnett - Curly, a rube
Paul Fix - The Handler
Ray Corrigan - Cheela the Gorilla
The film makes extensive use of footage from Universal's 1933 circus film The Big Cage, starring Clyde Beatty. Stone, normally a supporting player, was given a rare leading role because his physical resemblance to Beatty made it easier to match new footage to old.
The film had two sequels: Jungle Woman (1944, with Acquanetta) and The Jungle Captive (1945, with Vicky Lane). Ankers played the lead again in the first sequel.