At Manor Farm, all of the working animals are called to a meeting by Old Major, a withered but respected pig. He calls for a revolution against Mr. Jones and the rest of the humans who run the farm and force the animals to work for nothing more than what is needed to keep them alive. Old Major explains, "Man serves the interests of no creature except himself." The animals decide this will be a worthy change, and rule that any creature that stands upon two legs is an enemy, but all with four or wings are friends.
Old Major soon dies of old age, but the animals successfully revolt against Mr. Jones and claim Manor Farm (soon renamed Animal Farm) for themselves. The pigs, most intelligent of all farm animals, learn to read. Three of the most outstanding pigs, Snowball, Napoleon and Squealer, set out to educate fellow animals in the principles of "animalism", a way of living based on the Seven Commandments.
- Whatever goes upon two legs is an enemy.
- Whatever goes upon four legs or wings is a friend.
- No animal shall wear clothes.
- No animal shall sleep in a bed.
- No animal shall drink alcohol.
- No animal shall kill any other animal.
- All animals are equal.
The animals complete the hay harvest more successfully than they had when they worked under humans, but some animals notice that the pigs 'oversee' everything rather than work. On Sundays, a general assembly is held to plan the week ahead, Napoleon and Snowball put forth resolutions on which they almost never agree. Napoleon educates the newest litter of puppies in private, isolated from the rest of the farm.
Mr. Jones and his fellow farmers are all frightened by the rebellion and soon revolutions occur in nearby farms. Jones returns to reclaim his farm with a group of farmers, but the animals force their retreat. Snowball is injured, and one sheep is killed, but the reward "Animal Hero, First Class" is given to both Snowball and Boxer, the horse responsible for the most damage to humans. However, Mollie, a stubborn horse used to being pampered by humans, betrays Animal Farm to go live in the bordering human-run farm.
It is decided at the next general assembly that from now on the pigs shall make all final decisions. There is a great debate over Snowball’s new plans for a windmill, and when he seems to be holding the majority of the pigs' favor, Napoleon grows angry and releases his trained dogs that he has kept in isolation since birth. The dogs chase Snowball out of Animal Farm, and Napoleon takes charge. He ends the Sunday general assemblies, and rules that all decisions are to be made by a select committee of pigs, presided by himself. Any opposition to Napoleon’s new orders are silenced with harsh threats. He also decides that the windmill will, in fact, be built, and claims that it was actually his idea in the first place. No animal is brave enough to question this.
The quality of work decreases and the pace seems slowed after Snowball’s departure, even though work is required even Sunday afternoons if the animals do not want half rations. The animals grow uneasy when Napoleon begins associating and trading with humans in order to increase the rate at which the windmill is built. He claims that the resolution against trade and money never passed, or was even suggesting. Soon the Seven Commandments are once again violated when the pigs decide to reside in the farmhouse. Once again, Napoleon insists nothing restricting this sort of behavior was ever suggested. The fourth commandment is changed to "No animal shall sleep in a bed with sheets." Some of the animals seem to remember that it was different, but none are smart or strong-willed enough to oppose Napoleon.
Snowball is rumored to be hiding in a neighboring farm and is supposedly visiting Animal farm at night to upset food productions. Snowball becomes a "bogeyman" on which all bad fortune is blamed upon. Napoleon declares, "Snowball was in league with Jones from the very start!" He claims that Snowball was never in fact awarded with Animal Hero, First Class and insists that he, in fact, betrayed the farm and ran away cowardly during the battle against the humans. Boxer the horse questions Snowball’s betrayal, but is quickly silenced with threats from the dogs. Napoleon’s iron fist rule is displayed when four pigs are framed and killed for aiding Snowball. Three hens, three sheep and a goose are also killed in this manner.
The sixth Commandment is changed to "No animal shall kill any other animal without cause" in order to explain Napoleon’s injustices. Food production is worse than it was under the rule of Mr. Jones, but Squealer, Napoleon’s right hand man, lies about food figures. The mill is soon completed and named Napoleon Mill. Dealings with the humans grow hostile once more when Napoleon trades a large amount of timber to the farmer Fredericks, who pays with forged money. Animal Farm is attacked once more, this time by fifteen men with guns. The farmers capture the pasture and destroy the windmill. The animals retaliate but lose one cow, three sheep and two geese. Three men are killed by Boxer, the rest of the men flee when attack dogs threaten to surround them. Napoleon considers the battle a victory, despite the animal casualties. In an egotistic fit of celebration, Napoleon gets drunk and once again violates one of the Commandments. This fifth Commandment is changed to "No animal shall drink alcohol in excess,"
The rebuilding of the windmill begins, but Boxer, the most dedicated worker of all of the animals, suffers a split hoof, though he continues to work his hardest. The coming winter is a hard one; all rations are reduced except those for the pigs and dogs. Soon, pigs are given special rights by Napoleon. All barley is reserved for the pigs who receive a ration of beer daily. Animal Farm is declared to be a republic under the presidency of Napoleon (though there is in fact an election, the only candidate for presidency is Napoleon).
According to Animal Farm’s new president, Snowball apparently openly fought on Jones’ during the first battle against the farmers. The animals are still unable to oppose any resolutions the pigs come to, even though some of the quicker animals, such as the old goat, seem to remember that at one time Snowball hadn’t been an evil crook. Because of the hard winter and low rations, Boxer grows weaker and more tired, but insists on continuing his work on the windmill until he falls one day with a collapsed lung. He is supposedly sent to the best animal hospital in England, but the van that comes to pick him up is that of the town knacker.
Squealer announces that Boxer died in the hospital, and was not taken to the knacker’s, as rumor has it. Most animals believe this because they did not know how to read well enough to tell that the van was without a doubt the knacker’s, but a few animals begin to feel distrust towards Napoleon and his cohorts, even though they are helpless against their tyranny. Some resolutions that were previously passed to give animals of a certain age retirement homes are forgotten in hopes that the windmill will be completed sooner. Without Boxer’s help, the work progresses at a much slower rate, but the windmill is completed and used for milling corn. During this time Napoleon bought two more fields from the neighboring Pilkington Farm. However, animal education has been brought to a near halt; none of the new horses are able to learn the alphabet past B and all animals now accept everything that is said without a question.
The general situation of Animal Farm seems much like that of when it was a human owned farm, and the pigs slowly begin to resemble humans more and more. Every single one learns how to clumsily walk on his hind legs, and the malleable sheep who had previously chanted "Four legs good, two legs bad" now chant, "Four legs good, two legs better." Napoleon wears Jones’ old clothes left in the farmhouse and appears in all aspects to be a replica of Animal Farm’s former cruel ruler. One day, after a meeting with all of the nearby farmers, Animal Farm is renamed Manor Farm once again and all of the old commandments disappear although one statement is left where the others used to be:
All animals are equal, but some animals are more equal than others.