It's a true story!!!
Susanna Kaysen, an aimless 18-year-old from New England, overdosed on aspirin and vodka during a mental breakdown to kill herself in 1967. She is admitted to Claymoore, a local mental health facility, against her will. Susanna makes friends with several patients in the psychiatric ward, including Polly "Torch" Clark, a childlike woman with schizophrenia; Daisy Randone; Janet Webber, a sardonic woman with anorexia; Georgina Tuskin; Lisa, the psychopath who gets caught after she escapes the mental facility; and Cynthia Crowley. Susanna recalls her graduation day. When they called her name, she slept. During the checks, Susanna awakens amid the night and queries Georgina about herself. Georgina says that she is a pathological liar.
Again in the early morning, she gets a dream and recalls the time she spent that night at a pub. She has a casual relationship with Toby, a young man who has been enlisted to fight in the Vietnam War, in addition to her affair with Dr. Gilchrist. Lisa Rowe, who is charismatic and rebellious, particularly captivates Susanna.
At her subsequent counseling appointment, Susanna discusses that with Dr. Melvin. Susanna claims that she is still unsure of why they placed her in a mental facility. According to Dr. Melvin, she attempted suicide a week ago by taking aspirin and drinking a bottle of vodka. Lisa inspires her to quit taking her medicine and reject counseling. Later, Susanna and Lisa meet Daisy Randone, who self-harms, has obsessive-compulsive disorder, and is presumed to be bulimic. They exchange their medics. During regular treatment sessions with Dr. Melvin Potts, he initially hides her condition. The girls creep around at night in the hospital's underground tunnels and then go to Dr. Wick’s office, where Susanna discovers she has borderline personality disorder.
The women go to an ice cream shop as part of a rare supervised group outing to celebrate Daisy's upcoming release. Barbara Gilcrest, the spouse of the English instructor with whom Susanna had an affair, and their daughter Bonnie face Susanna there. Barbara publicly reprimands Susanna for having an affair with her husband; Lisa defends Susanna by attacking Barbara; the other girls make fun of Bonnie; and then Bonnie and Barbara leave feeling ashamed. Despite Valerie chastising Lisa, this makes Lisa even more endearing to Susanna.
Toby makes an appointment to see Susanna, and they have their private space in her room. Meanwhile, Lisa constantly irritates the staff, notably Valerie Owens, the harsh head nurse. When he leaves, he urges her to join him in fleeing to Canada. Susanna explains to him that she has gotten to know the other girls and would like to leave someday, but not with him. Additionally, Susanna had an affair with John, one of the hospital orderlies who expressed feelings for her. The following evening, Polly experiences a breakdown and is sequestered. Susanna and Lisa sedate the night watch nurse and sing to Polly to make her feel better. The two ladies are punished by Valerie when she discovers the group sleeping in the corridor in the morning. Lisa is subjected to electroshock therapy before being sent into solitary confinement. And John was moved to the men’s ward.
Later that evening, Lisa succeeds in escaping from her cell and persuades Susanna to join her. The women hitchhike to Daisy's recently rented flat, which her adoring father has provided, and offer her Valium in exchange for a place to stay the night. Lisa confronts Daisy after learning that she has been self-harming, despite Daisy's insistence that her condition has been treated. Daisy is made fun of by Lisa, who accuses her of relishing the incestuous sexual abuse Daisy has long endured at the hands of her father.
The next morning, Susanna discovers Daisy dead in her toilet after she hangs herself and slits her wrists. When Lisa looks for money in Daisy's room and on her body, Susanna is horrified. Lisa escapes to Florida. While Susanna calls for an ambulance and heads back to Claymoore after realizing she does not want to end up like Lisa. When she returns to the hospital, Susanna keeps herself busy with writing and painting while also participating in therapy, which includes frequent consultations with Dr. Sonia Wick, the institution's chief psychologist.
Before Susanna's release, Lisa is captured and brought back to Claymoore. One night, she steals Susanna's diary and reads it aloud to the patients in the tunnels, amusing them and making them hate Susanna. Lisa confronts Susanna and pursues her into the tunnels after Susanna reads a note in which she expresses pity for Lisa, who is described as a cold, gloomy person. Susanna confronts Lisa when she is cornered and claims that Lisa is terrified of the world, emotionally reliant on Claymoore, and dead inside.
Lisa loses it and considers killing herself, but the others can talk her out of it. Susanna visits Lisa, who is confined to a bed before she is let go the following day. After they make amends, Lisa claims that she is not truly heartless. They part ways amicably, and Susanna formally departs Claymoore after saying farewell to everyone.
