Plot

In 1963 Oregon, Randle Patrick "Mac" McMurphy (Jack Nicholson), a recidivist anti-authoritarian criminal serving a short sentence on a prison farm for statutory sexual assault of a 15-year-old girl, is transferred to a mental institution for evaluation. Although he does not show any overt signs of mental illness, he hopes to avoid hard labor and serve the rest of his sentence in a more relaxed hospital environment.

McMurphy's ward is run by steely, unyielding Nurse Mildred Ratched (Louise Fletcher), who employs subtle humiliation, unpleasant medical treatments and a mind-numbing daily routine to suppress the patients. McMurphy finds that they are more fearful of Ratched than they are focused on becoming functional in the outside world. McMurphy establishes himself immediately as the leader; his fellow patients include Billy Bibbit (Brad Dourif), a nervous, stuttering young man; Charlie Cheswick (Sydney Lassick), a man disposed to childish fits of temper; Martini (Danny DeVito), who is delusional; Dale Harding (William Redfield), a high-strung, well-educated paranoid; Max Taber (Christopher Lloyd), who is belligerent and profane; Jim Sefelt (William Duell); and "Chief" Bromden (Will Sampson), a silent American Indian believed to be deaf and mute.

McMurphy's and Ratched's battle of wills escalates rapidly. When McMurphy's card games win away everyone's cigarettes, Ratched confiscates the cigarettes and rations them out. McMurphy calls for votes on ward policy changes to challenge her. He makes a show of betting the other patients he can escape by lifting an old hydrotherapy console-a massive marble plumbing fixture-off the floor and sending it through the window; when he fails to do so, he turns to them and says, "But I tried goddammit. At least I did that."

McMurphy steals a hospital bus, herds his colleagues aboard, stops to pick up Candy (Marya Small), a party girl, and takes the group deep sea fishing on a commandeered boat. He tells them: "You're not nuts, you are fishermen!" and they begin to feel faint stirrings of self-determination. Soon after, however, McMurphy learns that Ratched and the doctors have the power to keep him committed indefinitely. Sensing a rising tide of insurrection among the group, Ratched tightens her grip on everyone. During one of her group humiliation sessions, Cheswick's agitation boils over and he, McMurphy and the Chief wind-up brawling with the orderlies.

They are sent up to the "shock shop" for Electro-Convulsive therapy. While McMurphy and the Chief wait their turn, McMurphy offers Chief a piece of gum, and Chief murmurs "Thank you." McMurphy is delighted to find that Bromden is neither deaf nor mute, and that he stays silent to deflect attention. After the electroshock therapy, McMurphy shuffles back onto the ward feigning illness, before humorously animating his face and loudly greeting his fellow patients, assuring everyone that the ECT only charged him up all the more and that the next woman to take him on will "light up like a pinball machine and pay off in silver dollars."

But the struggle with Ratched is taking its toll, and with his release date no longer a certainty, McMurphy plans an escape. He phones Candy to bring her friend Rose (Louisa Moritz) and some booze to the hospital late one night. They enter through a window after McMurphy bribes the night orderly, Mr. Turkle (Scatman Crothers). McMurphy and Candy invite the patients into the day room for a Christmas party; the group breaks into the drug locker, puts on music, and enjoys a bacchanalian rampage. At the end of the night, McMurphy and Bromden prepare to climb out the window with the girls. McMurphy says goodbye to everyone and invites an emotional Billy to escape with them; he declines, saying he is not yet ready to leave the hospital-though he would like to date Candy in the future. McMurphy insists Billy have sex with Candy right then and there. Billy and Candy agree, and they retire to a private room. The effects of the alcohol and pilfered medication take their toll on everyone, including McMurphy and the Chief, whose eyes slowly close in fatigue.

Nurse Ratched arrives the next morning and discovers the scene: the ward completely upended, and patients passed out all over the floor. She orders the attendants to lock the window, clean up, and conduct a head count. When they find Billy and Candy, the other patients applaud and buoyed, Billy speaks for the first time without a stutter. Nurse Ratched then announces that she will tell Billy's mother what he has done. Billy panics, his stutter returns, and he starts punching himself in the face; locked in the doctor's office, he kills himself. McMurphy, enraged at Nurse Ratched, chokes her nearly to death until orderly Washington knocks him out.

Sometime later, the patients in the ward play cards and gamble for cigarettes as before, only now with Harding dealing and delivering a pale imitation of McMurphy's patter. Nurse Ratched, still recovering from the neck injury sustained during McMurphy's attack, wears a neck brace and speaks in a thin, reedy voice. The patients pass a whispered rumor that McMurphy dramatically escaped the hospital rather than being taken "upstairs."

Late that night, Chief Bromden sees McMurphy being escorted back to his bed, and initially believes that he has returned so they can escape together, which he is now ready to do since McMurphy has made him feel "as big as a mountain." However, when he looks closely at McMurphy's unresponsive face, he is horrified to see lobotomy scars on his forehead. Unwilling to allow McMurphy to live in such a state-or be seen this way by the other patients-the Chief smothers McMurphy to death with his pillow. He then carries out McMurphy's escape plan by lifting the hydrotherapy console off the floor and hurling the massive fixture through a grated window, climbing through and running off into the distance, with Taber waking up just in time to see the Chief escape and cheering as the others awake.