Stanley Kubrick's adaptation of A Clockwork Orange opens with a close-up on Alex, the hard-edged protagonist and narrator of the film. He and his three "droogs" (friends), sit in the darkened Korova Milk Bar, drinking "milk-plus" (drug-laced milk), and getting ready to go out and commit some ultra-violence. Their first order of business is to beat up an old tramp, howling drunkenly beneath a bridge - simply because Alex abhors the idea of "a drunk old man".
Next, Alex and his droogs (named Pete, Georgie, and Dim) encounter Billy boy's rival group of droogs, who are about to gang-rape a young woman (or "devotchka") in a decrepit, abandoned casino. Instead of carrying out the rape, Billy boy and his droogs accept Alex's challenge to fight, and the 8 men descend upon each other; punching, kicking, and swinging heavy chains. The sound of cop cars echoes in the distance, and Alex and his droogs run off - leaving their opponents defeated and bloody on the ground.
Alex and his droogs manage to steal a convertible and drive recklessly into the darkened countryside, until they arrive at a remote house with a neon sign out front reading "Home". Alex goes to the front door and pretends to need a phone because his friend has been in an accident and is critically injured. The homeowners, Mr. and Mrs. Alexander, believe him. However, once Mrs. Alexander unlocks the door - Alex and his droogs put on masks and march inside gleefully, singing and whooping while destroying the Alexanders' home. They beat Mr. Alexander to the ground and force him to watch while Alex rapes Mrs. Alexander. Alex commits this "ultra-violence" while singing 'Singin' in the Rain'.
Later, Alex and his droogs go back to Korova Milkbar for a nightcap. There, a woman ("sophisto") sings a selection from Beethoven's 9th Symphony, and Alex is bewitched. Dim, however, is disrespectful and Alex raps his errant droog across the knees. The night then ends and Alex goes home to sleep. The next morning, his mother tries to wake him for school (he has been a truant all week) but Alex claims to be ill. After he awakens mid-day, he finds Mr. Deltoid, his post-correction officer, in his home. Mr. Deltoid, while attempting to molest Alex, warns the young man that his antics will land him in prison one day. Alex does not take Deltoid seriously.
Later, Alex goes to a record store to pick up a new Beethoven recording. While he is there, he convinces two young women to come home with him. Back in Alex's room, he engages in a multitude of sex acts with both women - the scene is filmed in fast motion. As evening sets in, Alex goes downstairs to the lobby of his apartment building, where he finds Pete, Georgie, and Dim waiting for him. Georgie speaks of a "new way" for their group - he and Dim want to abandon their usual itinerary of petty thievery for a bigger scam. Alex is not pleased with the threat to his power, and while the droogs walk along the marina, he throws Dim and Georgie into the water and beats them.
Later, after reasserting his leadership, Alex asks Georgie about his big plan for the night. Georgie describes a remote "health farm" run by a wealthy woman. It is shut down for the week and therefore, the woman (Cat Lady) will be there alone. At the Health Farm, Cat Lady is doing yoga, surrounded by her cats, when she hears Alex knock on the door. He claims that his friend has been hit by a car and he needs a phone - but Cat Lady refuses to let him in. She calls the police to report the incident. Meanwhile, the droogs help Alex through an open window and he sneaks into Cat Lady's room. She fights him, and he attacks her with a giant phallic sculpture, leaving her unconscious on the ground.
Alex rushes outside the house, hearing the police sirens approach. However, instead of following his orders to flee, Dim breaks a bottle of milk on Alex's face, and the droogs run off - leaving Alex alone, writhing on the ground in pain. In the police station, Alex finds out from Mr. Deltoid that Cat Lady died, which makes him a murderer. He tries to pin the crime on his droogs, but the police don't fall for his false claims of innocence. Alex is sentenced to 14 years in prison, and is now known as "Prisoner #655321".
Two years into his sentence, Alex has become close to the Prison Chaplain and reads the Bible regularly - although he really only enjoys the sections that describe the Crucifixion and other bloody battles. He acts like an ideal prisoner, desperate to do whatever he must to get out of prison as soon as possible. The Minister of the Interior comes to visit the prison, and singles Alex out for the experimental Ludovico technique - a treatment that supposedly cures criminals. It only takes a few days and guarantees Alex's freedom.
The treatment consists of injecting Alex with drugs, then strapping him down to a chair with his arms bound and his eyelids clipped open. He is forced to watch films of horrible violence, sexual crimes, and all kinds of sordid imagery, while the drugs make him violently ill. This aversion therapy is supposed to teach Alex's body to become critically ill whenever he thinks about criminal or sexual acts. However, one of the films - a montage of Nazi atrocities - is screened with Beethoven's 9th Symphony as its background score. It is only during this film that Alex screams for the treatment to stop, knowing that he will start feeling violently ill every time he hears Beethoven's 9th, but Dr. Brodsky (the psychiatrist in charge of the experiment) does not heed his pleas.
After Alex completes the course, the Minister of the Interior organizes a demonstration of the Ludovico technique. Alex stands on stage as an actor threatens and humiliates him - but Alex can't hit back because every time he feels the urge, he becomes violently ill. Then, a topless woman comes out and Alex tries to touch her breasts - but he crumples to the ground in pain. The Minister proclaims that the Ludovico technique is a success - despite the Prison Chaplain's criticism that the treatment deprives human beings of moral choice.
Alex arrives back home to the surprise of his Mum and Dad. However, they have rented out his room to a lodger named Joe - who claims to be like a son to them. Alex is therefore homeless, wandering the streets in misery. As he contemplates jumping into the river, he encounters the tramp he beat up earlier, and the tramp rallies his fellow tramps to beat Alex to the ground. Alex, of course, cannot hit back due to his treatment. Two cops rescue him from the tramps - and the cops turn out to be Dim and Georgie, Alex's old droogs. Dim and Georgie take the incapacitated Alex into the country and nearly drown him. Late at night, He stumbles through the rain, alone and confused, and finds himself at Mr. Alexander's door.
Mr. Alexander does not recognize Alex from the attack years earlier, but only from the newspaper articles about the Ludovico technique. He feels that Alex is a victim of the government's inhumane experiment and takes him in. While Alex is in the bath, Mr. Alexander calls his fellow Conspirators, excited about using Alex as a weapon against the current administration. However, he hears Alex singing "Singin' in the Rain" while bathing, and suddenly, Mr. Alexander realizes who Alex is.
After his bath, Alex sits down to eat a plate of spaghetti that Mr. Alexander has left out for him. Mr. Alexander and Julian, his bodyguard, sit on either side of him. Mr. Alexander tells Alex that his wife died from the same violent attack that left him crippled. Alex becomes uneasy and keeps drinking his wine. Soon, the Conspirators come and ask Alex questions about his treatment, and Alex explains that he cannot listen to Beethoven anymore, and he has become suicidal. With that, he passes out in his spaghetti. Alex wakes up locked in the upstairs bedroom of Mr. Alexander's home, while Mr. Alexander plays Beethoven's 9th as loudly as possible. Tortured, Alex jumps out the window of the house, landing on the pavement.
Alex wakes up in the hospital, having broken nearly every bone in his body. The media has since depicted him as a victim of the government's inhumane program to deal with Crime. Mum and Dad come to see Alex and show regret that they kicked him out. The Minister of the Interior then visits and says that Mr. Alexander has been imprisoned for trying to drive Alex to suicide. The Minister wants Alex on his side so that he can reverse the political fallout that has been occurring since Alex's suicide attempt. He has arranged for Alex to have a good job and start a new life. While being presented with a stereo system playing Beethoven and photographed for the papers, Alex imagines himself having sex with a woman on a bed of clouds, while stiffly-dressed men and women politely applaud. He proclaims, "I was cured, all right!"