It’s the account of Edward Prendick, an Englishman with a scientific education who survives a shipwreck in the southern Pacific Ocean and is rescued by a passing ship. He meets a man named Montgomery and his grotesque bestial native manservant M'ling. The ship is transporting a number of animals which belong to Montgomery. As they approach the island, Montgomery's destination, the captain demands Prendick leave the ship with Montgomery.
The island belongs to Dr Moreau, formerly an eminent physiologist in London who fled England when his gruesome experiments in vivisection had been publicly exposed.
The next day, Prendick runs into the jungle to avoid the awful cries of a pather that Moreau is experimenting on. While he wanders, he comes upon a group of people who seem human but have an unmistakable resemblance to swine. He is followed, and flees from a monstrous hybrid of animal and man, a Leopard-Man. He stuns him with a stone and escapes.
Prendick awakes the next morning with the previous night's activities fresh in his mind. He believes that Moreau has been vivisecting humans and that he is the next test subject. He flees into the jungle where he meets an Ape-Man who takes him to a colony of similarly half-human/half-animal creatures. Their leader is a large grey thing named the Sayer of the Law who has him recite a strange litany called the Law that involves prohibitions against bestial behavior and praise for Moreau.
Suddenly, Dr Moreau bursts into the colony looking for Prendick, but Prendick escapes to the jungle. He makes for the ocean, where he plans to drown himself rather than allow Moreau to experiment on him. Moreau explains that the creatures called the Beast Folk were not formerly men, but rather animals. Prendick returns to the enclosure, where Moreau explains that he has been on the island for eleven years and has been striving to make a complete transformation of an animal to a human. He explains that while he is getting closer to perfection, his subjects have a habit of reverting to their animal form and behaviour. Moreau regards the pain he inflicts as insignificant and an unavoidable side effect in the name of his scientific experiments.
One day, Prendick and Montgomery encounter a half-eaten rabbit. Since eating flesh and tasting blood are strong prohibitions, Dr Moreau calls an assembly of the Beast Folk and identifies the Leopard-Man as the transgressor. The Leopard-Man flees to avoid further vivisection. Eventually, the group corners him, but Prendick takes pity and shoots him to spare him from further suffering. Dr Moreau is furious that Prendick killed the Leopard-Man but can do nothing about it.
As time passes, Prendick becomes used to the grotesqueness of the Beast Folk. However one day, a half-finished puma woman rips free of its restraints and escapes from the lab. Dr Moreau pursues it, but the two end up fighting and kill each other. Montgomery, his servant M'ling, and the Sayer of the Law die after a scuffle with the Beast Folk. At the same time, Prendick knocks over a lamp and the compound burns down.
Prendick lives with the Beast Folk on the island for months after the deaths of Moreau and Montgomery. They increasingly revert to their original animal instincts, beginning to hunt the island's rabbits, returning to walking on all fours, and leaving their shared living areas for the wild.
Luckily for Prendick a boat that carries two corpses drifts onto the beach. Prendick uses the boat to leave the island and is picked up three days later. But when he tells his story he is thought to be mad, so he feigns amnesia.
Back in England, Prendick is no longer comfortable in the presence of humans who seem to him to be about to revert to the animal state. He leaves London and lives in near-solitude in the countryside, devoting himself to chemistry as well as astronomy in the studies of which he finds some peace.
[adapted from Wikipedia]