In 1963, Randle McMurphy is on an Oregon work farm for the statutory rape of a 15-year-old girl. He pretends to be mentally insane in order to get himself transferred to a mental institution and avoid hard labor. The ward is dominated by head nurse Mildred Ratched, a cold, passive-aggressive tyrant who intimidates her patients.
The other patients include young, anxious, stuttering Billy Bibbit; Charlie Cheswick, who is prone to temper tantrums; delusional, child-like Martini; the articulate, repressed homosexual Dale Harding; belligerent and profane Max Taber; epileptics Jim Sefelt and Bruce Fredrickson; quiet but violent-minded Scanlon; tall, deaf-mute Native American "Chief" Bromden; and several others with chronic conditions.
Ratched sees McMurphy's lively, rebellious presence as a threat to her authority, which she responds to by confiscating and rationing the patients' cigarettes and suspending their card-playing privileges. McMurphy finds himself in a battle of wills against Ratched. He steals a school bus, escaping with several patients to go fishing on the Pacific Coast and encouraging them to discover their own abilities and find self-confidence.
After an orderly tells him that the judge's sentence does not apply to people who are deemed to be criminally insane, McMurphy makes plans to escape, encouraging Chief Bromden to throw a hydrotherapy console through a window. It is also revealed that McMurphy, Chief, and Taber are the only non-chronic patients involuntarily committed to the institution; the rest of them are self-committed and could leave at any time, but are too afraid to do so. After Cheswick bursts into a fit and demands his cigarettes, which had been rationed by Ratched, McMurphy fights with the orderlies, and Chief intervenes.
Ratched sends Chief, Cheswick, and McMurphy to the "shock shop" as a result of this insubordination. While awaiting their punishment, McMurphy offers Chief a stick of gum, and discovers he can speak and hear, having feigned his deaf-muteness to avoid engaging with anyone. After being subjected to electroconvulsive therapy, McMurphy returns to the ward pretending to be brain damaged, but then reveals that the treatment has made him even more determined to defeat Ratched. McMurphy and Chief make plans to escape, but decide to throw a secret Christmas party for their friends after Ratched and the orderlies leave for the night.
McMurphy sneaks two women, Candy and Rose, and bottles of alcohol into the ward; he bribes guard Turkle to allow this. After the party, McMurphy and Chief prepare to escape, inviting Billy to come with them. Billy refuses, but asks for a "date" with Candy; McMurphy arranges for him to have sex with her. McMurphy and the others get drunk, and McMurphy falls asleep instead of making his escape with Chief.
Ratched arrives in the morning to find the ward in disarray and most of the patients passed out. She discovers Billy and Candy together, and aims to embarrass Billy in front of everyone. Billy manages to overcome his stutter and stands up to Ratched. When she threatens to tell his mother, Billy cracks under the pressure and reverts to stuttering. Ratched has him placed in the doctor's office. Moments later, McMurphy punches an orderly when trying to escape out of a window with the Chief, causing the other orderlies to intervene. Meanwhile, Billy commits suicide by slitting his throat with broken glass. Ratched tries to ease the situation by calling for the day's routine to continue as usual, and an enraged McMurphy strangles Ratched. The orderlies subdue McMurphy, saving Ratched's life.
Some time later, Ratched is wearing a neck brace and speaking with a weak voice, and Harding now leads the now-unsuspended card-playing. McMurphy is nowhere to be found, leading to rumors that he has escaped. Later that night, Chief sees McMurphy being returned to his bed. He greets him, elated that McMurphy had kept his promise not to escape without him, but notices McMurphy is unresponsive and physically limp, and discovers lobotomy scars on his forehead. Chief tearfully hugs McMurphy and says, "You're coming with me," before smothering him to death with a pillow. He then lifts the hydrotherapy fountain off the floor, smashes it through the window gates, and escapes alone, all while the remaining inmates, having been woken up by the glass breaking noise, watch and cheer him on.
Jack Nicholson as Randle Patrick "R.P." McMurphy
Louise Fletcher as Nurse Mildred Ratched
Will Sampson as "Chief" Bromden
William Redfield as Dale Harding
Brad Dourif as Billy Bibbit
Sydney Lassick as Charlie Cheswick
Christopher Lloyd as Max Taber
Danny DeVito as Martini
Dean Brooks as Dr. John Spivey
William Duell as Jim Sefelt
Vincent Schiavelli as Bruce Frederickson
Michael Berryman as Ellis
Alonzo Brown as Attendant Miller
Mwako Cumbaka as Attendant Warren
Nathan George as Attendant Washington
Marya Small as Candy
Scatman Crothers as Night Guard Turkle
Phil Roth as Woolsey
Louisa Moritz as Rose
Peter Brocco as Col. Matterson
Delos V. Smith Jr. as Inmate Scanlon
Josip Elic as Inmate Bancini
Mimi Sarkisian as Nurse Pilbow
Ted Markland as Hap Arlich
based on the 1962 novel of the same name by Ken Kesey