London, England, 1995. Alexander DeLarge (Malcolm McDowell) is seated in the Korova Milk Bar with his droogs Georgie (James Marcus), Pete (Michael Tarn) and Dim (Warren Clarke) drinking milk laced with narcotics. Alex's gang beats up an old wino (Paul Farrell) in a concrete tunnel. They arrive at a large defunct theater where another gang is about to gang rape a girl. Instead the two gangs have a battle. Next we see Alex and the group in a stolen Durango 95, heading around the countryside looking for action. They head out to a fancy house with a sign outside that says 'Home'. They convince the house's owner, a writer Mr. Alexander (Patrick Magee) to let his wife (Adrienne Corri) open the door by saying they were in an accident but they quickly tie him up and rape his wife while Alex sings 'Singin in the Rain'. The gang head back to the Corova Milk Bar. A woman seated at an adjacent table begins to sing Beethoven's 9th Symphony. Alex is ecstatic, but Dim makes a farting noise, so Alex hits him in the crotch with his cane. Dim is upset but Alex dismisses him and says they need to go home and sleep.
Alex walks home to his apartment, where he listens to Beethoven and masturbates. His mother tries to wake him up in the morning, but he keeps sleeping. We see him walking around his home in his underwear and he encounters his probation officer, P.R. Deltoid (Aubrey Morris), who threatens him with prison if gets in trouble again, hits on him, and accidentally drinks the water his dad's dentures are sitting in.
Alex goes out to a record store at a local shopping mall and brings two girls home with him. He has sex with both women in fast-motion while listening to the William Tell Overture. Later, he goes downstairs and encounters his droogs. They say they've been talking, and Pete has a plan to make lots more money by robbing a rich lady who lives at a dairy on the edge of town. Alex perceives this as a threat but goes along with the plan at first. Then he attacks his droogs as they walk down a pier, saying that the sound of Beethoven from a passing car inspired his violence. We see them in a restaurant and Alex once again brings up the dairy robbery plan.
At the dairy, they try the same trick they used on the writer, but the Catlady (Miriam Karlin) is suspicious and calls the police. Alex sneaks up the drainpipe and confronts her, striking her with a plaster phallic symbol just before police sirens are heard. He runs out the front door but the other droogs hit him in the face with a milk bottle and he is apprehended. At the police station he is questioned by several cops, and Deltoid shows up to spit in his face and tell him how disappointed he is. Alex laughs it off, but is soon headed for prison because the Catlady has died of her wounds.
Recieving a 14-year prison sentance, Alex checks all his possessions in with the head guard (Michael Bates) and is undressed and inspected for lice and venereal disease. He is also given a number, 655321. Two years later, Alex is seen doing about his duties including helping the prison chaplain (Godfrey Quigley) in his service. Alex is sitting in the library reading the Bible and imagining himself torturing Jesus at the crucifixion and killing people and laying about with his wives in an Old Testament setting. He speaks to the Chaplain and tells him that he's heard of a new treatment that helps people get out of prison and makes sure they don't return. The Chaplain says that this is only an experimental treatment and that hes not sure if its right for Alex. But the Minister of the Interior (Anthony Sharp) soon visits the prison and selects Alex as the perfect subject for this experiment. Alex is taken to the prison Governor's office (Michael Gover) and told that although the Governor would rather punish him, he's going to be leaving the prison.
The head guard transports Alex to the new medical facility where he is briefly inspected and a female doctor (Madge Branom) who promises him that hell be fine and gives him an injection. He's taken to a movie theater and strapped to a restraining chair with his eyes forcibly opened with clamps so that he cannot look away from the film, while a medical technician constantly puts eyedrops in his clamped open eyes to prevent them from drying out and to also keep Alex from passing out. They show him film of a rape and beating and he begins to feel sick to the stomach. He tells the doctor about it afterwards but she assures him that his feeling of sickness is a sign that he's returning to normal. He is later taken back to the theater and they show him more films, and this time they show him footage of Nazi soldiers while Beethovens 9th plays in the background. Alex recognizes the song and is horrified, screaming that such lovely music doesnt belong with these images. The male doctor (Carl Duering) speculates that the prison Governor would be pleased because this would be Alex's punishment before reassuring Alex that he will be cured within two weeks, with two shows like this a day.
Two weeks later, Alex is taken to an auditorium where he is paraded by the Minister in front of a group of dignitaries. He is confronted by an angry Irishman who throws him to the ground and is able to force him to lick his boot, which amuses the head guard. Next he is shown a lovely naked woman but collapses in nausea when he tries to touch her breasts.The Minister gets up and proclaims a new era in law enforcement and social justice, but the Chaplain says that the procedure has debased Alexs human nature by taking away his ability to actually choose good over evil.
Alex's parents sit in their living room reading about Alex's release in the newspaper. Alex enters the room and tries to be friendly, but there is a very awkward feeling in the room and finally he inquires about the man who is eating toast on the couch. His father says he is a lodger named Joe (Clive Francis) who cannot be kicked out because he's already paid next month's rent. Alex seems upset but Joe pushes him even farther, saying that it makes him sick to think of the things he did before going to prison. Alex tries to hit him but becomes sick again and storms out. We see him staring into a river holding a package that contains all the items taken from him when he went into prison. A bum asks him for change and Alex gives it to him, but the bum recognizes him as the same guy who beat him up in the tunnel two years earlier. Alex looks at him in horror and begins to try to escape but is trapped in the same tunnel by the bum and a lot of his elderly compatriots, who all hit and kick Alex as he cowers on the ground. Two policemen show up to break up the fight but it turns out that they are Dim and Pete, who have taken a job with the police. They take him out in a van to a wooded area outside town, walk into the woods, push his head in a water trough and hit him repeatedly with their batons, and leave him for dead.
Alex is able to crawl out of the woods and back into a rural neighborhood; we see him crawl past the same 'Home' sign that he drove past earlier. Alex is walking right into the house of Mr. Alexander whose wife he raped before he went to prison. A muscle-bound manservant, named Julian (David Prowse), lets him in and carries him into the den, where he recognizes the writer. But Mr. Alexander only recognizes him as the subject of the "Ludovico treatment," and invites him to have supper with him and take a bath. We see Mr. Alexander calling a friend and talking about how Alex is a victim of the government, and Alex is once again 'Singin' in the Rain' while he takes a bath. The writer recognizes the tune and realizes who Alex is, causing him to have a strange seizure. At the dinner table Mr. Alexander encourages Alex to drink wine and eat. Flanked by the wheelchair-bound Mr. Alexander and the burly Julian, Alex eats a plate of spagetti. As Alex eats, he grows more and more afraid of the hostile-looking Mr. Alexander, wondering if he knows who he really is. Then a man and a woman come to the house and ask Alex a series of questions about the Ludovico treatment and the fact that it has made him incapable of listening to Beethovens 9th Symphony as well as committing violence. He collapses in mid-sentence, drugged by the wine, and awakens in a strange small room on a bed. Beethovens 9th is being blasted from a stereo just below him, and the writer beams with joy as Alex pounds on the floorboards in pain and panic. Alex decides to attempt suicide as the only way to escape the music so he jumps from the window.
Alex wakes up in the hospital several days later with his whole body in a cast and observes a doctor and a nurse making out in the next stall. His parents bring him flowers and he blows them off. A female psychologist with blue hair comes to him to ask him a series of questions based on impressions of cartoons that she is showing him. Then he is visited by the Minister, who apologizes for what his government did to Alex and assures him that he wants to be friends with Alex if Alex will smile for the newspaper cameras, and that he will help Alex find a good job when he gets out. Alex milks it for all its worth, insisting that the Minister spoonfeed him while he makes his demands. The Minister says he has a present for Alex and assistants wheel in a giant stereo which is blasting classical music. As the newsmen come in and the flashlbulbs light up, Alex fantasizes about people in 19th Century costumes in an orgy in the snow and assures, "I was cured, all right."