Shipwrecked traveler Edward Parker is rescued by a freighter delivering animals to an isolated South Seas island owned by Dr. Moreau. After Parker fights with the freighter's drunken captain for his mistreating M'ling, a passenger with some bestial features, the captain tosses Parker overboard into Mr. Montgomery's boat, bound for Moreau's island .

When Parker arrives at the island, Moreau welcomes Parker to his home and introduces him to Lota, a young woman whom Moreau claims is of Polynesian origin, and who seems fearful and withdrawn. When she and Parker hear screams coming from another room, which Lota calls "the House of Pain," Parker investigates. He sees Moreau and Moreau's assistant, Montgomery, operating on a human-like creature without anesthetic. Convinced that Moreau is engaged in sadistic vivisection, Parker tries to leave, only to encounter brutish-looking humanoids resembling apes, felines, swine, and other beasts emerging from the jungle. Moreau appears, cracks his whip, and orders them to recite a series of rules ("the Law"). Afterward, the strange "men" disperse.

As Parker spends time with Lota, she falls in love with him. Eventually the two kiss, but Parker is then stricken with guilt, since he still loves his fiancée, Ruth Thomas. As Lota hugs him, Parker examines her fingernails, which are reverting to animalistic claws. He storms into the office of Dr. Moreau to confront him for creating Lota. Dr. Moreau explains that Lota is his most nearly human creation, and he wanted to see if she was capable of falling in love with a man and bearing human-themed children. Enraged, Parker punches Moreau to the ground and demands to leave the island. When Moreau realizes Lota is beginning to revert to her panther origin, he first despairs, believing that he has failed—until he notices Lota weeping, showing human emotion. His hopes are raised and he screams that he will "burn out" the remaining animal in her in the House of Pain.

Meanwhile, the American consul at Apia in Samoa, Parker's original destination, learns about Parker's location from the cowed freighter captain. Fiancée Ruth Thomas persuades Captain Donahue to take her to Moreau's island. She is reunited with Parker, but Moreau persuades them to stay the night. The ape-themed Ouran, one of Moreau's creations, tries to break into Ruth's room. She wakes up and screams for help, and Ouran is driven away. Montgomery confronts Moreau, and implies that Ouran's attempted break-in was arranged by Moreau. Donahue offers to try to reach the ship and fetch his crew. Moreau, seeing him depart, dispatches Ouran to strangle him.

Learning that Moreau has allowed Ouran to break the Law, the other beast-men no longer feel bound by it. They set their huts ablaze and defy Moreau, who tries and fails to regain control. He demands of them, "What is the Law?" Their response is, "Law no more!" The beast-men drag the doctor into his House of Pain, where they bind him, screaming, to the operating table and assault him with his own surgical knives.

With help from the disaffected Montgomery, Parker and Ruth make their escape. Parker insists they take Lota along. When Lota sees Ouran following, she waits in ambush. In the ensuing struggle, both are killed. The others escape by boat as the island goes up in flames, presumably destroying Moreau's work and eradicating the beast-men.