Nurse Ratched, in a black cape, walks into a locked ward of sleeping men. The first order of the day is medication, and the men line up at the nurse’s station to take their pills while the phonograph plays a soporiphic waltz. Like a burst of outside air, McMurphy arrives at the institution flanked by two guards. When they remove his handcuffs, he kisses one of the guards in sheer exuberance, cackling and bouncing with joy at being free.
On the ward, Nurse Ratched initially ignores McMurphy while she deals bureaucratically with his paperwork. McMurphy introduces himself to the Chief, a huge deaf-mute man sweeping the hall, then to Billy Bibbit—a stuttering mental patient with a fixation on his mother—and the other patients playing pinochle. McMurphy is loud and rambunctious, luring one patient away from the game with a deck of pornographic playing cards.
Dr. Spivey explores with McMurphy the reason he has been sent to the mental hospital from the prison work farm, where he was held previously. The doctor asks McMurphy whether he is faking mental illness to get out of work, and McMurphy admits slyly that he believes there is nothing wrong with his mind.
At McMurphy’s first group therapy session, he riffles his cards while Nurse Ratched speaks—his first gesture of defiance toward her authority. When the session deteriorates into shouting, Nurse Ratched remains straight-faced, impassively reacting at the group’s dysfunction. Later, she watches disapprovingly from a window as McMurphy tries to teach the Chief to play basketball. McMurphy also teaches the men to play blackjack. At one point, annoyed with the loud waltz music being played, he invades the nurse’s station to turn it down, thereby coming into direct conflict with Nurse Ratched, as patients are not allowed in the nurse’s station.
McMurphy’s conflict with Nurse Ratched erupts further during the World Series, when McMurphy proposes that Nurse Ratched revise the work schedule so the prisoners can watch the baseball games on television. She argues that the patients would find the change too disruptive. However, she agrees to a vote, knowing that most of the patients lack the courage to oppose her. When only two patients vote on his side, McMurphy is shocked. Later, he boasts to the men that he plans to go downtown to watch the World Series in a bar, betting that he can escape by lifting a marble water fixture and throwing it through the window. When he fails, he says that at least he tried.
The next day in group, Nurse Ratched mercilessly presses Billy Bibbit about a girlfriend, his mother, and his first suicide attempt until Cheswick comes to the hapless Billy’s defense. Cheswick joins McMurphy in demanding the World Series, and they force another vote. When all the men in group raise their hands, Nurse Ratched informs them that the “chronics”—the most severe, withdrawn patients—also must vote. McMurphy tries in vain to get any of these catatonic lost souls to respond, and Nurse Ratched adjourns the meeting. Belatedly, the Chief raises his hand to break the tie, but Nurse Ratched refuses to count his vote. Although she appears to have won, McMurphy sits in front of the dark television screen and begins to call the baseball game play-by-play. The other men join him in wonder, cheering imaginary hits and runs under his contagious enthusiasm. Nurse Ratched demands that they stop shouting, but for once she cannot control them.
Dr. Spivey asks McMurphy about his experience on the ward thus far, and McMurphy complains about Nurse Ratched. In response, Dr. Spivey tells McMurphy he sees Nurse Ratched as one of the finest nurses on the ward. Another doctor asks McMurphy how McMurphy’s perceptions of Nurse Ratched’s unfairness make him feel and what the maxim “moss doesn’t grow on a rolling stone” means to him. McMurphy offers his explanation. The conversation ends with McMurphy flippantly showing Spivey a picture of a naked woman from his deck of pornographic cards. He asks Spivey if he knows where the woman lives.
In his frustration at being imprisoned on the ward, McMurphy climbs over the hospital fence with the Chief’s help. He hijacks the bus to take the nonrestricted patients on an outing, picking up his girlfriend, Candy, along the way and driving the men to the docks. After boarding a fishing boat, McMurphy introduces the patients to a suspicious harbormaster, claiming that they are doctors from the mental hospital who have chartered the boat for a fishing trip. They motor out of the harbor, and McMurphy teaches Cheswick to drive the boat while the other men learn to fish. Taber catches a monster fish.
When the boat trip is done, the men return to face Dr. Spivey. The doctors decide that McMurphy is dangerous, and although Dr. Spivey wants to send him back to the prison farm, he defers to Nurse Ratched, who thinks McMurphy should stay in the institution. McMurphy later discovers that this means he is committed for as long as they think he should be—not the mere sixty-eight days left on his prison term.
Nurse Ratched suspends privileges and begins rationing cigarettes. The men question her authority, however, and she starts to lose control of the group. As Cheswick explodes in rage, McMurphy puts his hand through the glass of the nurse’s station to retrieve Cheswick’s cigarettes. When a fight breaks out between McMurphy and the lead attendant, Washington, the Chief comes to his defense.
In retaliation, Nurse Ratched sends Cheswick, McMurphy, and the Chief to electroshock therapy. As they wait, McMurphy offers the Chief a stick of Juicy Fruit gum, and the Chief thanks McMurphy, proving that he actually can speak and hear. When McMurphy reappears on the ward after his shock treatments, he rolls his eyes back and walks like a zombie to fool the men, then erupts in characteristic laughter to prove once again that he cannot be suppressed or dominated. Privately, however, he confesses to the Chief that he cannot take any more and plans to escape.
McMurphy bribes the night orderly so he can bring his girlfriend, Candy, her friend Rose, and some alcohol into the ward for a party. The men drink, play Christmas music, and dance with the girls. McMurphy removes the keys from the drunken orderly and says goodbye to the men. He invites Billy Bibbit to come with him, but Billy is not ready. Instead, McMurphy arranges for Billy to have sex with Candy, delaying his own escape. Everyone falls asleep, and in the morning Nurse Ratched finds the ward in disarray, with the window hanging open. When she discovers Billy Bibbit naked in bed with Candy, she invokes his mother’s name, making him disintegrate with shame. McMurphy tries to make it out the window, but a nurse’s scream alerts him that Billy has just committed suicide. McMurphy attacks Nurse Ratched, strangling her until Washington punches him senseless while she gasps for air. The guards take McMurphy away.
The Chief waits for McMurphy to come back to the ward. McMurphy returns in the night, wholly changed: he’s become a vegetable with lobotomy scars on both sides of his skull. Saying that he will take McMurphy with him, the Chief smothers his friend with a pillow. Then the Chief lifts the marble water fixture from the floor, throws it through the window, and escapes into the dawn.