Manor Farm is run by Mr Jones, a cruel and drunken farmer. One day, the animals gather at a meeting to listen to Old Major, a wise old pig. Old Major makes a speech, calling for animals to rise up against their farmers. The animals are very keen about the idea. Old Major dies a few days later. The pigs, who are the most intelligent animals, begin planning a rebellion. They are led by Snowball and Napoleon.
About three months later, the animals revolt against Mr Jones and take over the farm. They change its name to "Animal Farm". They decide the farm will now be ruled only by animals, calling the system "animalism". They write seven commandments, with the most important being "All animals are equal". “Four legs good, two legs bad” becomes a popular chant for the sheep. Mr Jones and his friends attack the farm to try to take it back, but in the "Battle of the Cowshed" the animals drive them away. Mr Jones flees and never returns.
Snowball and Napoleon fall out with each other, as they have different ideas for how Animal Farm should be led. Snowball announces plans to build a windmill, but Napoleon disagrees. Napoleon trains nine puppies, who grow into vicious dogs who will serve him. When the dogs are fully grown, he uses them to chase Snowball off the farm. He takes control and rules the farm by himself (as a dictator). Napoleon uses the dogs to kill any animal that does not agree with him, while a pig called Squealer keeps telling the animals that everything is fine and they should support Napoleon.
Napoleon changes his mind and decides to build a windmill, pretending that it was his idea all along. The first windmill they build collapses. Napoleon blames this (and other problems) on Snowball. He claims that Snowball is sneaking around Animal Farm ruining everything. Many animals are killed for "being in contact with Snowball". Napoleon starts working with human beings outside, even though this used to be forbidden. One of them is their neighbour, a farmer called Mr Frederick. He brings in a group of men who invade the farm and destroy the second windmill. The animals fight them off in the "Battle of the Windmill", at great cost.
As the animals build a third windmill, Boxer, their strongest horse, loses his strength because of old age and collapses. Napoleon sends him to be slaughtered, even though Boxer was his friend. The pigs continue to work with humans and start to behave like them, such as living in the farmhouse and walking on two legs. They teach the sheep a new chant: “Four legs good, two legs better”. The commandments are replaced by the words "All animals are equal, but some animals are more equal than others." In the end, other animals watch the pigs talking with a group of humans and decide they cannot tell which is which.
