The Forgotten (1973)

Nurse Charlotte Beale arrives at the isolated Stephens Sanitarium to work, only to learn that Dr. Stephens was murdered by one of the patients and his successor, Dr. Geraldine Masters, is not very eager to take on new staff. Charlotte finds her job maddeningly hard as the patients torment and harass her at every turn, and she soon learns why Dr. Masters is so eager to keep outsiders out.