In 1962, two versions of Anthony Burgess’s novel A Clockwork Orange were published. One concludes with Alex growing up and turning away from violence, while the second, darker version leaves out that final chapter. Kubrick based his film on this second version. The first version, published first in England, has twenty-one chapters, as Burgess intended it to. Burgess believed that all individuals, even those as violent as Alex, could reform, and in this version, moral growth comes with age. However, Burgess’s U.S. editor felt that the twenty-first chapter was “bland” and showed an unwillingness “to accept that a human being could be a model of unregenerable evil.” Later, Burgess said he originally agreed to publish the U.S. version without its final chapter only because he needed the money. Kubrick read the U.S. version of the novel and made his film without knowing about the twenty-first chapter. When he eventually read it, he still claimed to prefer the darker version of the story.

Aside from leaving out the twenty-first chapter, Kubrick stayed fairly close to the plot and language of Burgess’s novel. However, because film, unlike literature, impacts so many human senses and is so immediately tactile, Kubrick’s version heightened the controversy surrounding the novel’s violence. Kubrick introduced a number of details not found in the novel that further increased the story’s shock value. For example, the most famous moment of the film, when Alex sings “Singin’ in the Rain” while preparing to rape a woman, was Kubrick’s invention—it’s not in the novel. Several other plot differences between the film and the novel make Kubrick’s version darker. In the novel, the prison chaplain leaves his post to protest the government, but Kubrick does not include this ethical move in the film. Also, Kubrick makes Alex want to take part in Ludovico’s Technique so he can get out of prison quickly, while in the novel, Alex is forced to participate. Kubrick’s Alex is an accomplice to the state, while Burgess’s Alex, for all his violence, is clearly more of a victim.

Burgess praised the power of Kubrick’s film but in later years also complained about Kubrick’s omission of his intended ending. Burgess explained that he objected to his work being used to send a message that some humans are simply evil by nature. In an introduction to a revised U.S. version in which Burgess’s editor and publisher agreed to reprint the novel with the twenty-first chapter, Burgess wrote: “Much of my later life has been expended on Xeroxing statements of intention and frustration of intention—while both Kubrick and my New York publisher coolly bask in the rewards of their misdemeanor. Life is, of course, terrible.” Kubrick, however, continued to believe that the story was better without the final chapter, and the film and novel, though very similar in many ways, ultimately differ greatly in their final verdict on humanity.