In a futuristic Britain, Alex DeLarge is the leader of a gang of "droogs": Georgie, Dim and Pete. One night, after getting intoxicated, they engage in an evening of "ultra-violence", which includes a fight with a rival gang. They drive to the country home of writer Frank Alexander and trick his wife into letting them inside. They beat Alexander to the point of crippling him, and Alex violently rapes Alexander's wife while singing "Singin' in the Rain". The next day, while truant from school, Alex is approached by his probation officer, PR Deltoid, who is aware of Alex's activities and cautions him.
Alex's droogs express discontent with petty crime and want more equality and high-yield thefts, but Alex asserts his authority by attacking them. Later, Alex invades the home of a wealthy "cat-lady" and bludgeons her with a phallic sculpture while his droogs remain outside. On hearing sirens, Alex tries to flee, but Dim smashes a bottle in his face, stunning Alex and leaving him to be arrested. Deltoid brings word that the woman has died of her injuries, and Alex is convicted of murder and sentenced to 14 years in prison.
Two years into the sentence, Alex eagerly takes up an offer to be a test subject for the Minister of the Interior's new Ludovico technique, an experimental aversion therapy for rehabilitating criminals within two weeks. Alex is strapped to a chair, his eyes are clamped open, and he is injected with drugs. He is then forced to watch films of sex and violence, some of which are accompanied by the music of his favourite composer, Ludwig van Beethoven. Alex becomes nauseated by the films and, fearing the technique will make him sick upon hearing Beethoven, begs for an end to the treatment.
Two weeks later, the Minister demonstrates Alex's rehabilitation to a gathering of officials. Alex is unable to fight back against an actor who taunts and attacks him and becomes ill upon seeing a topless woman. The prison chaplain complains that Alex has been robbed of his free will; the Minister asserts that the Ludovico technique will cut crime and alleviate crowding in prisons.
Alex is released from prison, only to find that the police have sold his possessions to provide compensation to his victims and his parents have let out his room. Alex encounters an elderly vagrant whom he attacked years earlier, and the vagrant and his friends attack him. Alex is saved by two policemen but is shocked to find they are his former droogs Dim and Georgie. They drive him to the countryside, beat him, and nearly drown him before abandoning him. Alex barely makes it to the doorstep of a nearby home before collapsing.
Alex wakes up to find himself in the home of Mr Alexander, who is now using a wheelchair. Alexander does not recognise Alex from the previous attack, but knows of him and the Ludovico technique from the newspapers. He sees Alex as a political weapon and prepares to present him to his colleagues. While bathing, Alex breaks into "Singin' in the Rain", causing Alexander to realise that Alex was the person who assaulted his wife and him. With help from his colleagues, Alexander drugs Alex and locks him in an upstairs bedroom. He then plays Beethoven's Ninth Symphony loudly from the floor below. Unable to withstand the sickening pain, Alex attempts suicide by jumping out of the window.
Alex survives the attempt and wakes up in hospital with multiple injuries. While being given a series of psychological tests, he finds that he no longer has aversions to violence and sex. The Minister arrives and apologises to Alex. He informs Alex that the government has had Mr Alexander institutionalised. He offers to take care of Alex and get him a job in return for his co-operation with his election campaign and public relations counter-offensive. As a sign of goodwill, the Minister brings in a stereo system playing Beethoven's Ninth. Alex then contemplates violence and has vivid thoughts of having sex with a woman in front of an approving crowd, thinking to himself, "I was cured, all right!"
