Nurse Ratched isn't really concerned with curing her patients of their mental illness. For one thing, a number of men on the ward are free to discharge themselves from the facility at any moment. So the nurse figures that it would be better to create an environment in which she maintains maximum control. Every single aspect of day-to-day life on the ward, from the soporific string melodies to the group therapy sessions, is designed to keep the men in line and Nurse Ratched firmly in charge.

Unfortunately, this means that at no point are any of the patients ever able to deal with their mental health issues in a meaningful way. They are being cynically manipulated by Nurse Ratched, as we see most clearly during the group therapy sessions. That's why the arrival of McMurphy is such a threat to the power she has over the men. In his own way, he does offer the other patients a cure, albeit one that you won't find in any textbook of psychiatry.

The nurse has created an environment that is very controlled. Between the pills each patient takes, the music that plays, the meticulous schedule that must be followed, Nurse Ratchet creates a relationship of dependency between her and her patients. During group sessions, she uses the characters' weaknesses against them, driving each man more and more into himself. MacMurphy’s character is the antithesis of the dependent patients. His presence and antics create chaos to Ratchet’s controlled environment. The patients begin to act “normal.” She must finally control MacMurphy the only way she can; the essence of the character – the personality – is removed. The ward returns to the controlled routine it was at the beginning of the play. The Chief’s escape is the only sign the MacMurphy may have made a permanent change.