The film opens looking like the weirdest "Got Milk?" ad ever. Four teenagers drink milk in the Korova Milk Bar, a place decorated with tables that resemble naked women. The only thing missing is Austin Powers with a milk mustache.
This milk must be full of some very aggressive drugs, because after a glass the boys—our narrator Alex, Pete, Georgie, and Dim—leave and beat up a homeless man. Then they drive recklessly, knocking cars off the road. Finally, they arrive at an author's house, where they knock over his bookshelf, beat him, and rape his wife while belting "Singin' in the Rain." After that, it's back to the Korova Milk Bar…as if nothing happened.
The next day, Alex skips school and instead picks up two girls at a record store. He takes them back to his parents' house and has a ménage a trois with them. A different threesome is waiting for him in the lobby of his apartment building—his droogs, i.e., lackeys. They're tired of being lackeys, and Georgie and Dim want to take charge of the gang. Alex throws them into a river and even cuts one of them with a knife to show them who's the boss.
But the boys have a plan. They want to steal money from a woman who lives by herself way out in the woods. Alex thinks it's a good idea, so off they go. Alex sneaks into the woman's house and surprises her. She tries to fend him off, but he beats her to death with a giant porcelain phallus. Outside, Alex's droogs knock Alex out and leave him for the cops. Alex goes to jail.
In jail, Alex hears of a procedure that can expedite the release process. The Minister of the Interior chooses Alex to undergo the menacingly named Ludovico Technique. It's a simple procedure. Alex is injected with drugs, his eyelids are held open, and he's forced to watch snuff films.
Normally, this is Alex's idea of a fun Saturday night. But the drugs make him sick to his stomach, and condition him to be repulsed by violence. He is, in essence, "cured." However, during one of the films, Beethoven's Ninth Symphony is used as a soundtrack. This used to be Alex's favorite song—nothing wrong with loving Beethoven—and Alex can no longer listen to it without feeling ill.
Released from prison, Alex returns home to learn that his parents have rented out his bedroom. He leaves, wandering the streets. The homeless man he beat up recognizes Alex and drags him to an underpass, where a mob of homeless people violently returns the favor. Alex is rescued by the police, but the coppers happen to be his least-favorite droogs, Georgie and Dim, who take Alex into the woods and attempt to drown him.
Alex survives. Beaten and bloody he crawls to a nearby home—the home of the author whose wife he raped. The author is in a wheelchair from the attack, and his wife is dead. But the author doesn't recognize Alex, and takes him in.
Alex takes a warm bath, which prompts him to sing "Singin' in the Rain." Not only is this a rude, disruptive thing for a houseguest to do, but the author realizes who Alex is. Having read about Alex in the paper, the author knows Alex's aversion to Beethoven's Ninth Symphony. He blasts the music until Alex jumps out the window to get away from it.
But Alex survives. In the hospital, the Ludovico Technique is reversed. The government received lots of bad press for essentially brainwashing someone, so the Minister visits Alex for a positive photo-op. They play Beethoven, and Alex is able to listen to it without becoming ill. He shakes hands with the Minister and smiles for the camera. He feels cured. The end.