The film centers on Randle Patrick McMurphy, a criminal serving a prison term on a work farm. He is transferred to a mental institution due to his apparently deranged behaviour. In fact, McMurphy just feigns being mentally ill in the belief that he'll then be able to serve out the rest of his sentence in relative comfort and ease.
His ward in the mental institution is run by a calm but unfeeling tyrant, Nurse Ratched (Louise Fletcher). The other patients, most of whom are there voluntarily, are cowed into submission by her. Like McMurphy the patients hide in the ward from the law, their families, or the despair of their own lives. While McMurphy initially shows little respect for them, his antiauthoritarian nature is aroused. He takes Nurse Ratched on and undermines her rules and authority. Initially, it’s just for fun, but his sense of injustice at the treatment of the patients leads him into a real battle for their good. He animates the dull monotony with games, pranks, and excursions, but encounters stiff opposition from Nurse Ratched, whose system provides her with pills and electroshock to maintain control. What he finds out only later is that Ratched has the power to keep him there indefinitely.
McMurphy gradually forms deep friendships in the ward with a group of men which includes Billy Bibbit (Brad Dourif), a suicidal, stuttering teenager whom Ratched has humiliated and dominated, and "Chief" Bromden (Will Sampson), a huge Native American. Believed by the patients to be deaf and unable to speak, Chief is mostly ignored but also respected for his enormous size. In Billy, McMurphy sees a younger brother figure whom he wants to teach to have fun, while the Chief ultimately becomes his only real confidant.
Later, McMurphy, Chief and patient Charlie Cheswick (Sydney Lassick) are detained for being involved in a fight with the guards. Cheswick undergoes electroshock therapy, while McMurphy and Chief wait their turn on a bench. While they wait, McMurphy...