The narrator, 15-year-old Alex, and his gang - Dim, Pete, and Georgie - run amok in futuristic London. When the foursome isn't downing drug-laced milk in the Korova Milkbar and speaking in the Slavic-influenced slang of nadsat, they are robbing, beating, and raping socialist London's citizens. On this particular night, they beat up an old man with science books and a homeless man, get into a fight with a rival gang led by Billyboy, and steal a car and take it for a joyride to the country. At a cottage labeled "HOME," they beat up the author of "A Clockwork Orange" - a manuscript celebrating human free will and denouncing any infringement upon it - and rape his wife. Back at the Korova Milkbar, Alex hits Dim for interrupting a woman singing a piece from an opera - Alex is a great lover of classical music, especially Beethoven, and he always imagines himself engaging in violent and sexual acts while listening to it.
Alex's parents are ineffectual, and his farcical Post-Corrective Adviser, P.R. Deltoid, cannot fathom why London's youth has turned to criminality. The next night, Alex gets into a fight with Dim and Georgie to assert his leadership. The gang proposes they rob a rich old woman's house. After an unsuccessful attempt to get the woman to open the door, Alex sneaks into the house while his friends wait outside. He gets into a fight with the woman and her cats, but the police soon arrive. His friends betray him, temporarily blinding him while they flee, and Alex is arrested. The police brutalize Alex and are elated to have caught him. Alex soon discovers the woman has died, and he is sentenced to 14 years of jail for murder.
Alex, now known as number "6655321," spends two years in State jail, dealing with brutal wardens, homosexual prisoners, and mindless labor. He relates that Georgie has died. His one supporter in prison is the chaplain, who has taken Alex under his wing since Alex got interested in the Bible - little does he know that Alex entertains violent fantasies when reading the book. Alex asks about a new treatment - Ludovico's Technique - which frees the prisoner and ensures he remains free. The chaplain is skeptical about the treatment, as it eliminates the subject's power to choose. A cell scuffle results in Alex's killing a new prisoner, and the powerful Minister of the Interior asks the prison Governor to use Alex as a guinea pig for the new treatment.
Alex shrugs off the chaplain's concerns about the treatment and signs up. He is transferred to a new hospital, where he is given a shot after each filling meal. The treatment, under Dr. Brodsky, consists of being forced to watch violent films (his eyelids are propped open) while strapped in a chair. The films are violent, and Alex has a terrible physical reaction to their violent content, feeling sick and begging the doctors to stop. The doctors have a sadistic streak in them, however, and happily continue the treatment. Alex soon finds even the thought of violence, not to mention the demonstration of it in reality, makes him ill. Classical music, used as a soundtrack for some of the films, also makes him sick by association. After two weeks, Alex's treatment is over and he is trotted out to demonstrate the effects for an audience. Even without the shot, any semblance of violence or sex debilitates him, and he is pronounced cured by the Minister.
Alex, now a free man, is also a celebrity, his case touted by the Government as a major step in turning back rampant crime. He finds London is a less violent place now. He is no longer welcome in his home, as a lodger named Joe has displaced him in his parents' home. Alex no longer enjoys classical music, either, and contemplates suicide. The old man with the science books and other elderly people beat Alex up in the library, only for the police - now including Dim and Billyboy - to take Alex into the country and further brutalize him. They leave him for dead, and he seeks shelter in the "HOME" cottage.
The man there, F. Alexander, knows Alex from the newspapers and takes him in. A liberal, he wants to use Alex to overthrow the totalitarian Government. He mentions that his wife was raped and killed, victimized in much the same way Alex has been. Alex is careful not to let the man know he was the rapist, but his use of nadsat slowly establishes that fact in the F. Alexander's mind. He and his friends go to work on Alex's case, and in the meantime put him in an apartment in the city. However, they set it up so Alex must listen to sickness-inducing classical music near an open window. Alex jumps out to commit suicide, realizing the men have betrayed him so his suicide can help their cause against the Government.
Alex survives and is put in a hospital. When he comes to, F. Alexander's friends tell him he has destroyed the Government's chances for re-election. He drifts out of consciousness again and when he next comes to, his parents beg him to return to their home; Joe has left after some trouble with the police. Alex is regaining his tolerance for violence, and after a few days he is back to where he started, the effects of Ludovico's Technique apparently reversed by doctors in his sleep. The Minister of the Interior stages a photo opportunity in which he gets Alex to denounce F. Alexander - who has been put away after learning Alex raped and killed his wife - and befriend the Government.
Alex forms a new gang and, with his cushy new Government-supplied job, seems to be renewing his former life. But he finds that drugs and violence no longer excite him, and he has even developed a taste for romantic, as opposed to violent, classical pieces. When he sees that his old friend Pete has become a middle-class husband, it seals the deal: Alex wants to settle down, marry, and have a son. He believes he has simply outgrown his violent past. It was youth, above all, that made him, and all the sons in the world, act impetuously.