After McMurphy returns from the successful fishing trip, Nurse Ratched begins an open attack on McMurphy. She tries to convince the patients that McMurphy is conning them and that everything he does is for his own gain. Nurse Ratched is so successful with her criticisms that only the Chief and a couple of the others still believe in him. Even the Chief almost loses his faith in McMurphy over an incident with the control panel.
However, McMurphy soon redeems himself. When the orderlies are administering an enema to the patients, George, refuses to have one. McMurphy supports him and gets into a fight; he is willing to accept his punishment for all the patients. The Chief comes to his aid and although they win the fight, they are both punished. They are taken to the Disturbed ward for shock treatment. The Chief tells McMurphy about his childhood. He remembers a game that his grandmother used to play with him; the game had a rhyme about “one who flew over the cuckoo’s nest”, a foreshadowing that the Chief will flee from the hospital, the cuckoo’s nest. When he returns to the ward, he is a changed man. Because of McMurphy, the Chief feels strong and is determined to resist the fog.
McMurphy is given four shock treatments. His treatment is filled with religious imagery. The table where the shock is administered is in the form of a cross to which the patient is strapped. McMurphy jokes about the salve by saying, “Anointest my head with conductant. Do I get a crown of thorns?” He is seen as a martyr, willing to sacrifice himself for the good of the others. McMurphy is also given a hose to bite on during the treatment, just as Christ was given a piece of cloth dipped in vinegar. There is even a man on the ward who says, “I wash my hands of the whole deal,” referring to Pontius Pilate’s washing his hands of Christ’s trial. By the time he returns to the ward, he is a hero to the patients, a fact that really angers Nurse Ratched. She threatens him with a lobotomy, foreshadowing what will eventually happen to McMurphy. McMurphy still feels self-assured and thinks that the Nurse can’t touch him if he behaves.
The patients know that McMurphy will be targeted from now on, so they arrange an escape for him. McMurphy, however, is not ready to leave and he has decided to have a party, where Billy will lose his virginity. McMurphy bribes the night watchman and arranges for Candy to be there and for her to bring alcohol. Candy brings a friend, Sandy. The patients drink, have a great time, and lose control. McMurphy, who has too much to drink, doesn’t seem to care. Harding tries to make the others realise that the consequences of this party are going to be serious. He even tries to convince McMurphy to immediately escape.
They are all discovered the next morning by Nurse Ratched. Billy has fallen asleep with Candy. Ratched threatens to tell Billy’s mother about his behaviour. Billy is so upset at this thought that he slits his throat. Ratched blames McMurphy for his death. In retaliation McMurphy tears open her uniform, exposing her breasts to all the patients. She is rendered defenseless in front of all the patients. Ratched can no longer completely control the patients but she is still in control of McMurphy’s future. Ratched has him sent away to be dealt with severely. While he is gone, most of the patients, now strong enough to face the outside world, discharge themselves from the hospital.
When McMurphy comes back to the ward, only the Chief, Scanlon, and Martini remain and they quickly see that a lobotomy has been performed on McMurphy. Ratched intends to use him as an example to other patients. The Chief cannot bear to see McMurphy in such a state. He puts a pillow over McMurphy’s face and suffocates him. The Chief then escapes by throwing the control panel through the window, just as McMurphy has taught him to do. By killing McMurphy and escaping, the Chief proves that he has recovered; he also wins the final battle with Nurse Ratched.