Randle Patrick McMurphy (Nicholson), a criminal who has been sentenced to a fairly short prison term, decides to have himself declared insane so he'll be transferred to a mental institution, where he expects to serve the rest of his term in (comparative) comfort and luxury.

His ward in the mental institution is run by an unyielding tyrant, Nurse Ratched (Fletcher), who has cowed the patients, most of whom are there by choice, into dejected institutionalized submission.
McMurphy becomes ensnared in a number of power games with Nurse Ratched for the hearts and minds of the patients. All the time, however, the question is just how sane any of the players in the ward actually are.