The narrator, Edward Prendick, explains that he intends to recount the strange and horrible tale of the year he was lost at sea and presumed dead. His tale begins with the shipwreck of the Lady Vain off the coast of South America. Prendick and two other passengers board a dingy and float at sea for days without food or water. Growing desperate, the three men entertain thoughts of cannibalism. Prendick’s companions fight each other, and both fall overboard to their deaths, leaving Prendick alone on the dingey. Seemingly doomed and growing delirious, Prendick is scooped up and rescued by a passing schooner.  

Prendick regains consciousness several days later in the cabin of Montgomery, a passenger who has been caring for him. Prendick also encounters Montgomery’s attendant M’ling, a timid man with a dark complexion and a strangely proportioned body who arouses an inexplicable sense of revulsion in Prendick and others. The schooner is carrying a bizarre cargo of animals, including rabbits, staghounds, llamas, and a puma. The animals, which belong to Montgomery, are being ferried to the island where Montgomery and M’ling live. The ship’s captain, a raving drunkard, despises Montgomery, the animals, and above all M’ling, whom he and the rest of the crew consider a freak. Prendick also becomes a target of the captain’s rage when he attempts to defend Montgomery and M’ling.   

When the ship reaches the island, Montgomery disembarks with his animal cargo. The captain refuses to allow Prendick to stay on the ship, and Montgomery eventually agrees to let Prendick accompany him to the island. There, Prendick meets some of Montgomery’s men and notices that they have strange, unnaturally proportioned bodies and a bizarre skin tone. Montgomery introduces Prendick to Dr. Moreau, a white-haired man who explains that the island is a biological research station. Prendick recognizes Moreau’s name and recalls that, years ago, a  scientist by that name had been hounded out of England for his gruesome experiments on animals. Prendick’s misgivings are later confirmed when he overhears the tortured cries of an animal, which he assumes to be the puma, apparently being vivisected in the room next to his. Disturbed by the puma’s cries, Prendick flees out of the compound and into the jungle.  

In the jungle, Prendick encounters a strange man walking on four legs and lapping water from a stream like an animal. He tries to return to the compound, but the strange man stalks him. Fearing for his life, Prendick fashions an improvised sling and knocks the man unconscious. Back at the compound, he again hears tortured cries in the next room and suspects that Moreau is vivisecting a human. Barging through the door, he discovers Moreau vivisecting a creature bound to a rack. Fearing that Moreau plans to vivisect him next, Prendick flees again into the jungle. There, he meets a friendly Ape-man who offers him to food. The Ape-man takes Prendick to a cavern, where he lives with several other creatures who seem to be part human, part animal. Their leader, who introduces himself as the Sayer of the Law, leads these “Beast Folk” in the recital of “the Law,” a series of quasi-religious prohibitions of animal-like behaviors such as walking on all fours, eating flesh, and chasing other “men.” These rites seem designed to remind the animals to behave like “men” rather than animals, lest they be punished and subjected to great pain by Moreau, whom they revere as a sort of fearful deity.   

Montgomery and Moreau arrive at the cavern and order the creatures to capture Prendick. Prendick flees through the jungle and runs into the sea, determined to commit suicide rather than become Moreau’s next victim. But Moreau convinces Prendick that he means him no harm. Back at the compound, Moreau explains that he is researching “the plasticity of living forms.” His experiments aim to transform an animal into a human by vivisecting it, reconfiguring its body parts, combining it with parts from other animals, altering its body chemistry, and implanting certain ideas into its mind. So far, he has only succeeded in creating Beast Folk with some human qualities and abilities, but they all succumb to their animal instincts in ways that disappoint and frustrate Moreau. The Beast Folk on the island are all Moreau’s experimental creations, whom he casts away whenever he finds them insufficiently human.  

Later, in the jungle, Prendick and Montgomery discover the mutilated corpse of a rabbit and realize that some of the Beast Folk are deviating from the Law. Moreau decides to find and punish the perpetrator as an example. He summons all the Beast Folk and accuses the Leopard-man of breaking the Law. The Leopard-man attacks Moreau and then runs off into the jungle. The Beast Folk and humans give chase, eventually cornering the Leopard-man. But in a moment of sympathy, Prendick shoots the Leopard-man rather than allowing him to be captured and tortured by Moreau.  

The men uneasily return to the compound. A few weeks later, the puma breaks free from its restraints and bolts away. Moreau pursues the puma but fails to return. Montgomery goes out to find him and discovers that the Beast Folk have gone wild and killed Moreau. Drunk and despairing, Montgomery goes to the beach, where he  too is murdered, leaving Prendick the lone human on the island.  

Prendick survives for ten more months on the island by asserting authority over the Beast Folk. He forms an alliance with the Dog-man, who seems predisposed to serve him, and fends off challenges from others by brandishing weapons and issuing empty threats that prove effective on the simple-minded Beast Folk. Still, he lives in constant fear of the Hyena-swine, who refuses to serve him.  

As time passes, Prendick notices that all the Beast Folk are gradually regressing, losing their human features and abilities. One day, a small boat washes ashore. Prendick fills it with supplies and departs from the island. After several days, he is picked up by a ship. He tells no one his tale, fearing that he will be thought insane. He returns to England, where he is haunted by his memories of the island. Unable to assimilate back into civilized society, he retires to the countryside and seeks solace in the solitary study of astronomy.