Alex, a 15-year-old living in near-future England, leads his gang on nightly orgies of opportunistic yet random violence. Alex's friends ("droogs" in the novel's Anglo-Russified slang) are Dim, a slow-witted bruiser who is the gang's muscle, Georgie, and Pete. Alex, who is quick-witted and possessing an often disconcerting sense of humour, is clearly the smartest of the group and even seemingly cultured.


The novel begins with the droogs sitting in their favourite milkbar, drinking drugged milk to hype themselves for the night's mayhem. They assault a scholar walking home from the library, stamp on a panhandling derelict, scuffle with a rival gang led by Billyboy, rob a newsagent and leave its owners unconscious, then steal a car. Joyriding in the countryside, they break into an isolated cottage and maul the young couple living there, beating the husband and raping his wife. The droogs ditch the car, and Dim and Georgie make clear their dissatisfaction with Alex's domination of the gang. At home in his dreary flat, Alex plays classical music thunderously while bringing himself to climax with fantasies of even more orgiastic violence.


Alex skips school the following morning and is visited by P. R. Deltoid, a "post-corrective advisor" assigned to remediate his juvenile delinquency. Visiting his favourite music shop, Alex meets a pair of 10-year-olds and takes them back to his parents' flat, where he feeds them alcohol and sexually assaults them while they are intoxicated.


Alex later chats with his parents, who are skeptical of his claims about having a night job, yet too intimidated to press the issue. Arriving late to meet the droogs, who have already pumped themselves up with "the old knifey moloko" (i.e., drugged milk), Alex is at a disadvantage. Georgie challenges Alex for leadership of the gang, demanding that they pull a "man-sized" job by robbing a wealthy old woman who lives alone with her cats. Alex quells the rebellion by slashing Dim and Georgie in a knife fight, then in a show of generosity takes them to a bar for some fortifying drinks. Georgie and Dim are ready to call it a night, but Alex bullies them into proceeding with the burglary. At the woman's house, Alex enters through a second-floor window and, after a farcical struggle, knocks the old woman unconscious. When he tries to flee, Dim stops him and attacks him with his "oozy" (a length of chain) and the droogs leave Alex incapacitated in the doorway as they run off, after which Alex is roughed up by the police. The next day he finds out that the woman has died and he is to be charged with murder.