The poorly-run Manor Farm near Willingdon, England, is ripened for rebellion from its animal populace by neglect at the hands of the irresponsible and alcoholic farmer, Mr. Jones. One night, the exalted boar, Old Major, holds a conference, at which he calls for the overthrow of humans and teaches the animals a revolutionary song called "Beasts of England". When Old Major dies, two young pigs, Snowball and Napoleon, assume command and stage a revolt, driving Mr. Jones off the farm and renaming the property "Animal Farm". They adopt the Seven Commandments of Animalism, the most important of which is, "All animals are equal". The decree is painted in large letters on one side of the barn. Snowball teaches the animals to read and write, while Napoleon educates young puppies on the principles of Animalism. To commemorate the start of Animal Farm, Snowball raises a green flag with a white hoof and horn. Food is plentiful, and the farm runs smoothly. The pigs elevate themselves to positions of leadership and set aside special food items, ostensibly for their personal health. Following an unsuccessful attempt by Mr. Jones and his associates to retake the farm (later dubbed the "Battle of the Cowshed"), Snowball announces his plans to modernise the farm by building a windmill. Napoleon disputes this idea, and matters come to head, which culminate in Napoleon's dogs chasing Snowball away and Napoleon declaring himself supreme commander.
Napoleon enacts changes to the governance structure of the farm, replacing meetings with a committee of pigs who will run the farm. Through a young porker named Squealer, Napoleon claims credit for the windmill idea, claiming that Snowball was only trying to win animals to his side. The animals work harder with the promise of easier lives with the windmill. When the animals find the windmill collapsed after a violent storm, Napoleon and Squealer persuade the animals that Snowball is trying to sabotage their project and begin to purge the farm of animals Napoleon accuses of consorting with his old rival. When some animals recall the Battle of the Cowshed, Napoleon (who was nowhere to be found during the battle) gradually smears Snowball to the point of saying he is a collaborator of Mr. Jones, even dismissing the fact that Snowball was given an award of courage while falsely representing himself as the main hero of the battle. "Beasts of England" is replaced with "Animal Farm", while an anthem glorifying Napoleon, who appears to be adopting the lifestyle of a man ("Comrade Napoleon"), is composed and sung. Napoleon then conducts a second purge, during which many animals who are alleged to be helping Snowball in plots are executed by Napoleon's dogs, which troubles the rest of the animals. Despite their hardships, the animals are easily placated by Napoleon's retort that they are better off than they were under Mr. Jones, as well as by the sheep's continual bleating of “four legs good, two legs bad”.
Mr. Frederick, a neighbouring farmer, attacks the farm, using blasting powder to blow up the restored windmill. Although the animals win the battle, they do so at great cost, as many, including Boxer the workhorse, are wounded. Although he recovers from this, Boxer eventually collapses while working on the windmill (being almost 12 years old at that point). He is taken away in a knacker's van, and a donkey called Benjamin alerts the animals of this, but Squealer quickly waves off their alarm by persuading the animals that the van had been purchased from the knacker by an animal hospital and that the previous owner's signboard had not been repainted. Squealer subsequently reports Boxer's death and honours him with a festival the following day. (However, Napoleon had in fact engineered the sale of Boxer to the knacker, allowing him and his inner circle to acquire money to buy whisky for themselves.)
Years pass, the windmill is rebuilt, and another windmill is constructed, which makes the farm a good amount of income. However, the ideals that Snowball discussed, including stalls with electric lighting, heating, and running water, are forgotten, with Napoleon advocating that the happiest animals live simple lives. Snowball has been forgotten, alongside Boxer, with "the exception of the few who knew him". Many of the animals who participated in the rebellion are dead or old. Mr. Jones is also dead, saying he "died in an inebriates' home in another part of the country". The pigs start to resemble humans, as they walk upright, carry whips, drink alcohol, and wear clothes. The Seven Commandments are abridged to just one phrase: "All animals are equal, but some animals are more equal than others." The maxim "Four legs good, two legs bad" is similarly changed to "Four legs good, two legs better." Other changes include the Hoof and Horn flag being replaced with a plain green banner and Old Major's skull, which was previously put on display, being reburied.
Napoleon holds a dinner party for the pigs and local farmers, with whom he celebrates a new alliance. He abolishes the practice of the revolutionary traditions and restores the name "The Manor Farm". The men and pigs start playing cards, flattering and praising each other while cheating at the game. Both Napoleon and Mr. Pilkington, one of the farmers, play the Ace of Spades at the same time and both sides begin fighting loudly over who cheated first. When the animals outside look at the pigs and men, they can no longer distinguish between the two.