Pathologist Dr. Alix Pardee learns about an American doctor who has been experimenting on criminals to relieve them of their violent tendencies. She travels to the Phillippines and finds the doctor's secret laboratory in the jungle, but comes to find out that the experiments actually turn the men into beasts who are then released into the jungle as prey for the financier of the project to hunt like wild animals. Now privy to their terrible secret, Dr. Pardee is held captive by the two men, but eventually outsmarts them both.

At a plantation on a remote jungle island in the Philippines, American expatriot Dr. Bill Fleming conducts experiments in the hope of discovering a serum that will cure criminals of their psychopathic inclinations. One day, after several murders are committed in Manila by a man with animal-like features, Dr. Alix Pardee, a renowned pathologist, begins an investigation. As she travels through the islands to find the jungle from which the man-beast emerged, her boat plunges over a waterfall. Some time later, she awakens from unconsciousness to find herself at Bill's clinic. Although he tells Alix that he has been successful in his quest for the serum, he also admits that, occasionally, the criminals turn into hideous man-beasts. Some time later, Alix learns that, although Bill is sincere, his experiments are being financed by Stewart Victor, an oil tycoon who finds amusement in setting loose the man-beasts in the jungle and hunting them for sport. When Victor discovers that Alix knows his secret, he decides to kill her, but in the meantime, Bill accidentally takes some of his own serum and turns into an apeman. Before Victor can kill Alix, he is confronted by Bill, who kills him. Later, at the plantation, Alix discovers that the latest batch of Bill's serum is a complete success and decides to take it back to civilization, leaving the mutated Bill to wander the jungles alone.