The film begins with shadowy images on a wall showing a man being attacked by what appears to be a woman. He produces a needle from his pocket and administers its contents to his assailant. She falls to the ground, apparently dead.
The scene shifts to the coroner’s inquest into the death of Paula Dupree (Acquanetta), allegedly at the hands of Dr. Carl Fletcher (Naish). Induced to reluctantly tell his story by the coroner (Hinds) and district attorney (Dumbrille), Fletcher admits that he killed Paula. Yet, he also offers that there is more to the tale than what is on the surface.
Via flashbacks, it is learned that Fletcher was present on the opening night of the Whipple Circus and witnessed Cheela the Gorilla's heroic act that saved the life of Fred Mason (Stone). After her supposed demise, the doctor acquires the body of the beast. He detects a heartbeat, and revives the animal. Fascinated by the creature, he purchases the late Dr. Walters’ estate, hoping to find the records of his experiments.
Apparently having no need of more glandular material or another brain, Cheela returns to human form. She is at first unable to speak and Fletcher diagnoses her condition as being due to shock. However, upon the arrival of his daughter Joan (Collier) and her fiancé Bob Whitney (Richard Davis), the girl suddenly becomes verbal identifying herself as Paula.
Enamored with Bob, Paula’s jealous streak resurfaces. During a moonlight canoe ride, an unseen attacker capsizes Joan and Bob. Discussing the episode with Dr. Fletcher, they believe that one of the other patients Willie (Edward M. Hyans, Jr.) is responsible. Unbeknownst to them, Willie has already paid the supreme price for his constant annoyance towards Paula.
Later, meeting with Bob in private, Paula shows him bruises on her shoulder, injuries she claims were inflicted by Dr. Fletcher. Meanwhile, the caretaker for the estate has made the doctor aware of the vicious killing of his dog and a flock of chickens. Armed with the broken lock to the henhouse, Dr. Fletcher confiscates Paula’s perfume bottle as well. After he returns to his study, Paula makes an attempt on his life. Just as he pushes her to the floor, Bob enters the room. Misunderstanding what he has seen, he takes Paula to another doctor for an examination.
Dr. Fletcher has the lock and perfume bottle analyzed by a fingerprint expert, learning that although different in size, the prints do indeed match. He is now convinced that Paula and Cheela are one and the same.
Bob has Paula examined by Dr. Meredith (Pierre Watkin), who does detect mental instability and extreme physical strength. When told she is already under the care of a doctor, he admonishes the young man to return her at once to her attending physician.
Arriving back at Crestview, Dr. Fletcher is met with the news that Willie’s mangled body has been found. He confides in Joan what he knows about Paula, and expresses his concern for Bob.
Paula and Bob return as well, and the latter finally tells her that he is going to marry Joan, which visibly angers her. Joan rushes to meet him as Paula disappears in the shadows. Learning the truth about the girl, he sends his fiancée to her cabin while he goes to the aid of Dr. Fletcher.
After a search of Paula’s room fails, Dr. Fletcher tells Bob to check the rest of the house. Brandishing a hypodermic needle filled with a sedative, the physician heads outside to continue his search.
Paula pursues Joan through the woods to the cottage. Attempting to gain entrance, she hears the approaching Dr. Fletcher. She attacks him, and in the struggle, he accidentally administers a fatal overdose.
The flashbacks conclude and the D.A. expresses his disbelief. The Coroner parades the group, including the jury, to the morgue to re-examine the body of Paula Dupree. She is found to have reverted in death to the form of a half-human, half-ape monster. Dr. Fletcher is exonerated.Acquanetta as Paula Dupree
Evelyn Ankers as Beth Mason
J. Carrol Naish as Dr. Carl Fletcher
Samuel S. Hinds as Coroner
Lois Collier as Joan Fletcher
Milburn Stone as Fred Mason
Douglass Dumbrille as District Attorney
Richard David as Bob Whitney
Nana Bryant as Miss Gray
Pierre Watkin as Dr. Meredith
Christian Rub as George
Alec Craig as Caretaker
Eddie Hyans as Willie
Tom Keene as Joe
Jungle Woman was later followed by The Jungle Captive.