Manor Farm is a small farm in England run by the harsh and often drunk Mr. Jones. One night, a boar named Old Major gathers all the animals of Manor Farm together. Knowing that he will soon die, Old Major gives a speech in which he reveals to the animals that men cause all the misery that animals endure. Old Major says that all animals are equal and urges them to join together to rebel. He teaches them a revolutionary song called "Beasts of England." Old Major dies soon after, but two pigs named Snowball and Napoleon adapt his ideas into the philosophy of Animalism. They set about trying to spread Animalism’s ideals to the other animals on the farm, but this proves to be an uphill battle. The carthorses, Boxer and Clover, prove to be their best disciples, as they’re able to distill Animalism into simple arguments and share them with the other animals.
Three months later, Mr. Jones neglects to feed his animals for more than 24 hours. The animals revolt and chase Mr. Jones and the farmhands off of the farm in what ends up being an easy victory. The animals promptly burn all items that allowed Mr. Jones to maintain power, such as whips, bits, and knives. The next morning, the animals tour the farm and the pigs reveal that over the last few months, they’ve taught themselves to read. Snowball is the best at writing, and with white paint he amends the farm’s gate to read "Animal Farm." At the big barn, Snowball also writes the tenets of Animalism, which he and Napoleon distilled into Seven Commandments. The commandments state that all animals are equal, and no animal may act like a human by sleeping in a bed, walking on two legs, killing other animals, or drinking alcohol. They state that humans are the only enemy. The animals turn to the hay harvest after the pigs figure out how to milk the cows, but the milk begins to disappear.
The absence of humans means that the animals are far more successful than Mr. Jones ever was. There’s enough food, and the animals take pride in being able to feed themselves with their own labor. The pigs are clever enough to figure out how to perform certain tasks without standing on two legs, while Boxer seems as strong as three horses and adopts the motto “I will work harder!” All the animals throw themselves into the running of the farm except for the vain horse Mollie, who makes lots of excuses as to why she can’t work. Benjamin the donkey seems not to care about anything and cryptically tells everyone that donkeys live a long time.
Snowball organizes committees for the animals—which are mostly unsuccessful—and more successfully teaches animals to read. The dogs, the pigs, the goat Muriel, and Benjamin are the only ones who become fully literate. Less intelligent animals, such as the sheep, only learn the letter A and cannot remember the Seven Commandments, so Snowball distills this down into the maxim “Four legs good, two legs bad.” He has to explain to the birds why this is acceptable, since they have only two legs. Napoleon, meanwhile, takes the nine new puppies to train, insisting it’s more useful to focus on educating the young. A fight for power soon develops between Snowball and Napoleon.
Snowball and Napoleon send out pigeons to neighboring farms to spread the word to other animals. The other farmers sympathize with Mr. Jones, but only want to make the situation work for them. Fortunately for the animals, their neighbors, Mr. Pilkington of Foxwood Farm and Mr. Frederick of Pinchfield Farm, hate each other, though they’re both terrified of what happened at Animal Farm. In October, Mr. Jones and some men invade the farm with a gun. The animals fight bravely and send the men racing away, though Boxer is distraught when he believes he killed a stable boy. Snowball gives a speech about the importance of dying for Animal Farm and they agree to fire Mr. Jones’s gun twice per year, on the anniversaries of the rebellion and of the Battle of the Cowshed. They also come up with military honors and confer one on Snowball.
In the winter, Mollie disappears to serve a man in town. The pigs argue over how to plan the coming season and the rivalry between Snowball and Napoleon comes to a head over Snowball's idea to build a windmill. Snowball convinces animals by insisting that a windmill would give them electricity and ensure they only have to work three days per week, while Napoleon quietly insists this is nonsense. At the final debate about the windmill, Napoleon summons the puppies, whom he secretly reared to be his own vicious servants, and has them chase Snowball from Animal Farm. Napoleon tells the other animals that Snowball was a "bad influence," eliminates the animals' right to vote, and takes "the burden" of leadership on himself. He sends around a pig named Squealer, who persuades the animals that Napoleon has their best interests at heart.
Three weeks later Napoleon decides they should build the windmill after all—the windmill, he insists, was his idea to begin with, but Snowball stole his plans. The animals set to work, with Boxer leading. Focusing on the windmill reduces the productivity of the farm, and all the animals but the pigs and the dogs get less to eat. Napoleon institutes work on Sundays that’s voluntary, but animals who don’t work will receive reduced rations. The pigs engage a solicitor named Mr. Whymper to represent them and begin to trade with other farms. They move into Mr. Jones's farmhouse and start to sleep in beds. This confuses Clover, who thought this was forbidden. When she asks Muriel to read her the Commandment about beds, it reads: "No animal shall sleep in a bed with sheets." Squealer, accompanied by dogs, insists that if the pigs don’t get enough sleep, Mr. Jones will return.
That winter, a storm destroys the partially complete windmill. Napoleon blames the catastrophe on the "traitor" Snowball and insists that Snowball is hiding out at Foxwood. Humans insist that the windmill fell because of the weather and though the animals don’t believe it, they build the walls of the second windmill twice as thick. Napoleon covers up that the farm doesn’t have enough food, and in January, tells the hens that he’s agreed to trade 400 eggs per week for grain. The hens are distraught, as they’d all planned on hatching spring chicks, so they revolt and sacrifice their eggs. Napoleon cuts their rations and the hens give up after five days, after nine hens die. Napoleon circulates that they died of disease and catches wind that Snowball is sneaking onto Animal Farm and causing mischief, such as trampling eggs and stealing. One evening, Squealer insists that Snowball is in league with Mr. Frederick and has been on Mr. Jones’s side the whole time. Boxer is dumbfounded and notes that Snowball fought with them, but Squealer insists that according to Napoleon, Snowball is on Mr. Jones’s side.
Four days later, Napoleon sets his dogs on four young pigs and Boxer during a meeting. Boxer paws the dogs away, but the dogs rip the pigs’ throats out after they confess to conspiring with Snowball. Other animals confess heinous crimes as well, and the dogs kill all of them. The remaining animals gather at the windmill, and Boxer suggests that this happened because they’ve done something wrong. Clover can’t formulate her thoughts into words, but she thinks that this wasn’t what she had in mind when she joined the rebellion. However, she still thinks that this is better than living under Mr. Jones and vows to accept Napoleon’s leadership. She leads the animals in a round of “Beasts of England,” but Squealer stops by and announces that the song is now banned: the revolution it speaks of has happened, so it’s no longer useful. Minimus the pig composes a new song that none of the animals like as much. A few days after the massacre, Clover remembers that the Seven Commandments stated that animals shouldn’t kill each other, but when she asks Muriel to read the Commandments on the barn, the Commandment reads that animals can’t kill each other without cause.
The animals work harder than ever, and Squealer regularly reads them figures that show the farm’s productivity is up by 200 to 500 percent. Napoleon stays inside the farmhouse most of the time, guarded by the dogs. When Minimus composes a poem in Napoleon’s honor, Napoleon has it written on the barn next to the Commandments and a portrait of himself. Napoleon negotiates with Mr. Frederick and Mr. Pilkington about timber on the property he’d like to sell, and tensions run high. They finish the windmill in the fall and soon after, Napoleon announces he sold the timber to Mr. Frederick after promising it to Mr. Pilkington. The money will buy the animals the machinery for the windmill. Mr. Whymper, however, reveals that Mr. Frederick paid for the timber with forged banknotes. Mr. Frederick and his men, many with guns, invade Animal Farm and blow up the windmill. The enraged animals chase them away but feel discouraged until Squealer points out that they achieved a great victory. The pigs discover a case of whiskey and after initially announcing that Napoleon is dying, they declare that all spare fields will be planted with barley. All of it will go to the pigs. One night, animals hear a crash and find Squealer next to the barn with a broken ladder and paint. The next morning, the Commandments read that animals shouldn’t drink to excess.
As Boxer approaches retirement, he refuses to take time to let his injuries heal. He wants to see the windmill done. When 31 piglets, all Napoleon’s children, are born in the spring, Napoleon announces that they need to build a schoolhouse and institutes a rule that all other animals must let pigs pass. Napoleon is unanimously voted to be the farm’s president when it becomes a republic. In the summer, Boxer collapses while working on the windmill, and Napoleon announces that a human vet will treat him. When the van comes to collect Boxer, however, Benjamin rouses everyone: the van reads that Boxer is going to the glue factory. They never see Boxer again, and Squealer insists that the van was recently purchased by a vet and hadn’t yet been repainted. The pigs come up with money to buy more whiskey a few days later.
Years pass. Now only a few of the remaining animals on the farm experienced the revolution. Even fewer remember its goals. They complete the first windmill and begin a second, but neither windmill will electrify the farm. The pigs teach themselves to walk on two legs, begin carrying whips, and teach the sheep to bleat “Four legs good, two legs better.” When Clover and Benjamin check the Seven Commandments, they only see the statement: "All animals are equal. But some animals are more equal than others." The pigs make peace with their human neighbors and have a feast, but both Napoleon and Mr. Pilkington cheat at cards and begin a fight. The other animals are shocked to discover that they can no longer tell the pigs from the humans.