This movie is about a repeat offender named McMuphy (Nicholson) who is sent to prison for statutory rape. He convinces the prison guards the he is insane so that he will be transferred to a mental institution and be able to get out of prison labor. He assumes, wrongly, that that the institution will be the lap of luxury compared to prison. However, the institution is run by the notorious Nurse Ratched who as broken the spirits of all of the patients and forced them into submission through humiliation and fear of lobotomies. The rest of the movie is about their battle for control of the ward.
We're introduced to the key players in the movie over a game of cards, which is one of the main ways everyone passes their time. Dourif is very nervous and stutters uncontrollably, Cheswick becomes enraged easily and often throws tantrums, Harding is neurotic and well-educated, Taber can be obnoxious and is crude, and the Chief is an enormous Native American who is believed to be deaf and mute. As the movie progresses McMurphy, Dourif, and the Chief become close. We learn through there relationships that many of the patients are there voluntarily and that each has adopted a role or greater degree of "insanity" in order to cope and exist on the ward with some semblance of peace.
What is interesting about this film is how these "disabled" patients exhibit ableism. McMurphy, because of his "ability" and claimed greater degree of "sanity" assumes a leadership position of the patients. He assumes because of his abilities that he can assume that position. He continues to make these assumptions throughout the film. He uses the Chief to gain an advantage in basketball, but assumes that he is not good for anything else as he is supposedly deaf and mute. This is the basis of his relationship with Dourif as he seeks to impart wisdom.
The penultimate example of ableism, however, is when the Chief kills McMurphy at the end of the movie after he has been lobotomized. McMurphy was made into a "vegetable" and the Chief didn't want to leave him like that. The Chief who pretended to be deaf and mute for most of his time on the ward - an able individual passing as disabled - considered McMurphy's life as no longer worth living. His ability afforded him the right to make that decision and to implement his perspective. Playing devil's advocate, McMurphy very well may not have wanted to "live like that" but that decision would have been made while he was situated in his ableist privilege. As a "vegetable" McMurphy could have said he wanted to live or die, but that opinion, if able to be communicated, would have had no value as it came from disabled mind. The whole scene reaffirms that if you don't have a certain level of cognition or intellectual ability your thoughts and desires are irrelevant because you're not capable of having thoughts or desires with any degree of merit.