An animal trainer Fred Mason (Milburn Stone) returns 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'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). Beth reflects on 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 was discussion 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. 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.

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.