A man - he is also the narrator of the story - finds himself in a lifeboat with two other men after their ship, the Lady in Vain, has sunk. After some days drifting through the sea the two other men start to struggle for water, fall over board and drown. The narrator himself is saved by a ship called the Ipecacuahna, a little trader from Arica and Callao in South America.

When Prendick, that is the narrator`s name, regains consciousness he finds himself in a small cabin. There is a man who has looked after him while he was unconscious, his name is Montgomery. Prendick is so far recovered that he is able to leave the room. He learns that the ship is bound for Hawaii, but that it has to land Montgomery on a small island without a name first. When they leave the cabin they encounter a misshapen man, short, broad and clumsy, with a crooked back, a hairy neck and a head sunk between his shoulders. Also his face is deformed and in his huge mouth there are big white teeth. Then they go up to the deck of the ship. It is full of animals, there are staghounds, a puma, some rabbits and a llama, but Montgomery doesn`t tell Prendick what they are for. Then the strange misshapen man appears again. He is persued by a red haired man who ill-treats him. Montgomery tells the red haired drunk man, who is the captain of the ship, not to assault his passengers and they start to quarrel. The next morning they reach the island and Montgomery and his men are unloading the deck. The drunk captain is happy that they leave his ship and forces Prendick to leave the ship too. Montgomery and the others don`t want him to go to the island so the captain throws him into a lifeboat and cuts the rope. Prendick drifts away but then Montgomery takes pity and rescues him by picking him up with the landingboat. There is a white-haired man on board, who seems to be the the most important of them, but the others all look very strange, like the misshapen man they have met before, and they seem to be mere servants of Montgomery. They land. The white haired man tells Prendick that this is some sort of a biological station and that there passes a ship only every twelve month. There is an enclosure of coral and lave and Prendick gets a room with one door to the inside and one to the outside of the enclosure. The white haired man - Prendick learns by accident that his name is Moreau - tells him not to go inside the enclosure and locks the inner door. Then Prendick remembers where he has heard the name Moreau before: Moreau once was a great physiologist, but he had to leave England because of his cruel experiments with animals. Then Montgomery and Prendick take lunch and they can hear the howling of the Puma. Prendick can`t stand it any more and goes for a walk into the forest. There he encounters a half-beastial man drinking at the river and then disappearing in the bushes. Later, Prendick finds a killed rabbit and then he encounters three of these beastial looking men dancing and reciting some gibberish. He leaves the grotesque looking creatures and makes himself on the way back to the enclosure. It is getting dark and suddenly he realizes that he is persued by a creature and so he starts to run in panic and is finally able to reach the enclosure. There he meets Montgomery, who was looking for him, but he doesn`t want to tell Prendick what these creatures are.

The next day Montgomery comes through the inner door into Prendick`s room to check if he is alright, but then forgets to lock the door when he leaves. There is still the sound of cries in the air, not those of a puma but those of a human. Then, thinking that Moreau is vivisecting a human beeing Prendick enters the yard behind the inner door. There is blood everywhere and he can see Moreau and something bound painfully upon a framework, red and bandaged. Then he is pushed back into his room and the door is locked. Prendick is sure now that Moreau is vivisecting human beeings and that all the strange creatures are victims of his experiments. Very shocked he runs away into the jungle, where he has been on his previous expedition. He runs to the sea and plans to drown himself but then wants to find out more about the island. He then meets an ape-like creature which can speak a little. Prendick asks where he can find food and the creature takes him to their huts. There he meets different looking creatures and also the leader of them who is called "The Sayer of the Law".He is asked if he wants to live with them and as he answers yes he has to repeat a strange formula which was their "Law". They all start to sway:" Not to go on all Fours; that is the Law. Are we not Men?" In the same way they sway not to suck up drink, not to eat flesh nor fish, not to claw bark of trees and not to chase other men. Then they sing "His is the House of Pain", "His is the Hand that makes", "His is the Hand that wounds", "His is the Hand that heals", and so on. Prendick thinks that Moreau, after animalizing these men, has infected their dwarfed brains with a kind of deification of himself. As they are repeating the same phrases again and again suddenly all of the creatures jump up and line up. Moreau and Montgomery appear. Moreau tells the creatures to catch Prendick but he is able to escape. He now knows that he is not secure on this island, nor at Moreau nor at the Beast People. He goes along the beach and plans to break into the enclosure to get a weapon but suddenly his persuers appear in front of him and, sure that Moreau will animalize him, he walks into the sea to drown himself. But Moreau and Montgomery convince him that he was not secure among the Beast People and that they wanted to save him and that they won`t torture him. They throw away their guns and Prendick leaves the water and picks them up.

Back at the enclosure Moreau shows Prendick the puma, terribly cut and mutilated. He explains that all the creatures on the island are not humans that were animalized but animals that were humanized by surgery. They are animals carven and wrought into new shapes. Moreau transplants tissue from one part of the animal to another, or from one animal to another, to alter ist chemical reactions and methods of growth, to modify the articulations of ist limbs and to change it in ist most intimate structure. By hypnotism he taught the creatures to speak. Moreau has been humanizing animals for 20 years but he was never satisfied with his results. The beasts were not intellegent enough, although their bodies were almost human-like their intellegence stayed that of onimals. And after some time, he says, they revert, become animals again. Moreau has implanted some fixed ideas - the Law - into their brains to prevent that they attack humans. Montgomery likes the creatures quite much, he visits them regularely in their huts.

Next morning Montgomery and Prendick go for a walk into the forest and find a killed rabbit and Prendick tells Montgomery that that he has seen a dead rabbit and one of the cratures drinking in the river at his arrival on the island; the Beast People start to neglect the Law! They go back to tell Moreau about it.They all go to the huts and ask who has broken the Law and suddenly a Leopard Man - he has killed the rabbits - jumps up and runs away.There is a big tumult because all the creatures persue him. When they have wretched the poor creature into a corner Prendick shoots him out of pity, because otherwise he would have to go back to "The House of Pain". Prendick realizes how especially cruel Moreau`s experiments are, not only the torment in the enclosure but also the life after it. When they were animals, their instincts were adapted to their surroundings and now they stumble in the shackles of humanity, frightened by a law they could not understand. Prendick can`t understand the object of Moreau`s cruel investigations, he seems so irresponsible and utterly careless to him.

Then happens a catastrophe: the puma escapes and Moreau and Montgomery persue him. Montgomery is attacked by two Beast People who he shot - never before have the Beast People a human. When Prendick and Moreau go to look for Moreau the find his dead body. Now the Beast People that there is no Law any more but Prendick convinces them that Moreau is still watching them. They go back, lock themselves up in the enclosure and kill all creatures in the laboratory. Montgomery gets mad and drunk of the Brandy and leaves the enclosure. After a while Prendick sees that the boathouse is on fire - Montgomery has burned it to prevent their returen to mankind. Montgomery has been killed by the creatures. Also the enclosure is on fire because Prendick has overturned the oillamp when he has left it.

Prendick succeeds in making the Beast People obey him and so he lives ten more month on the island. During those ten month they are becoming dumber and dumber, they forget how to speak and to go erect, until they look almost like animals again. He builds a raft and leaves the island because the situation was growing more and more dangerous for him because the beasts started to neglect the Law. After three days he is picked up by a ship and saved. He decides not to tell anyone of what has happened to him on the island because nobody would believe him. Now men appear as strange to him as the Beast Folk on the island.