London, the near future. Teenager Alex DeLarge is obsessed by violence, rape and the music of Ludwig van Beethoven. He is the leader of a gang of young toughs, or 'droogs', Pete, Georgie and Dim, who begin a typical evening in the Korova Milk Bar before going out to beat a helpless tramp senseless in a subway underpass and have a turf war with a rival gang.

Wearing disguises, Alex, Pete, Georgie and Dim drive to the countryside and invade the home of the famous writer Mr Alexander. They tie him up and force him to watch the gang rape of his wife, performed to a choreographed dance based on the song 'Singin' in the Rain.

While his parents go to work, Alex plays truant, listens to his beloved Beethoven, is lectured by his probation officer P.R.Deltoid, picks up a couple of girls in a record shop and has sex with them in his bedroom. He then asserts his authority on Pete, Georgie and Dim, who vow revenge.

The quartet invade the home of a wealthy cat-loving eccentric, though when she dies as a result of Alex's assault, he is betrayed by his fellow droogs. He is given a long prison sentence, though he is offered the chance of freedom by the visiting Minister of the Interior if he agrees to subject himself to a new experimental process called the Ludovico Technique.

This involves being strapped to a chair with eyelids fixed open and staring at a screen on which is projected graphic images of sex and violence. The aim is for Alex to become nauseated through over-exposure, and the technique succeeds, but it also makes him repelled by the music of Beethoven, inadvertently chosen as the films' soundtrack.

Alex is released from prison, but finds himself ostracised by his parents, beaten up by the tramp he previously assaulted, and nearly drowned by his former droogs Georgie and Dim, who have since become police officers.

He is rescued by Mr Alexander, who fails to recognise him, and when he hears about Alex's suffering at the hands of the authorities, he vows to give his case maximum publicity with the aim of discrediting the government. However, his mood changes when he realises who Alex is after hearing him sing 'Singin' in the Rain', locks him in an upstairs room and tortures him by playing Beethoven at full volume. Alex tries to commit suicide by throwing himself out of the window.

Alex is taken to hospital, where he is both physically and mentally cured. He agrees not to press charges against the government after he realises that he is back to what he considers normal.