Set in a private mental institution, Chestnut Lodge in Rockville, Maryland, the film tells of a trainee occupational therapist, a troubled ex-soldier named Vincent Bruce (Beatty), who becomes dangerously obsessed with seductive, artistic, schizophrenia patient Lilith Arthur (Seberg). Bruce makes progress helping Lilith emerge from seclusion and leave the institutional grounds for a day in the country and accompanies her on other excursions in which she is alone with him. She attempts to seduce him, and eventually Bruce tells Lilith he is in love with her. Lilith also seduces an older female patient and enchants a couple of young boys on one of her outings. Bruce triggers the suicide of another patient (Fonda) out of jealousy over the patient's crush on Lilith. This brings up memories in Lilith of her brother's suicide, which she implies was due to an incestuous relationship that she initiated, and she goes on a destructive rampage in her room and winds up in a catatonic state. Bruce presents himself to his superiors for psychiatric help.