It's 1967, and a compulsive writer 17 year old girl named Susanna Kaysen (Winona Ryder) is like a lot of American teenagers of her age; confused, insecure, struggling to make sense of the rapidly changing world around her. But she had suicide intent, she mixed a bottle of aspirins with vodka, so pressed by her parents, she went to a psychiatrist.
The psychiatrist she meets with, however, gives to her behavior a name:
Borderline Personality Disorder. This disorder is manifested by uncertainty about self-image, long-term goals, types of friends or lovers to have, which values to adopt, a pervasive feeling of emptiness. Her self-concept is unstable and chaotic, she has a fragile identity, she seeks support and definition from others, and this turns in anger when is not helpful.
Her personality disorder lands her in a mental hospital named Claymoore'. Here, in between the pill dole from the nursing staff, she writes endlessly in her dog-eared journal and fills it with tell-tale drawings and she also loses herself in a world of eccentric young women, among them Daisy (Brittany Murphy), a pampered "Daddy's girl" with a predilection for rotisserie chicken and laxatives; and Polly (Elisabeth Moss), a burned victim whose heart, unlike her face, remains remarkably unscarred. Of course, she also latches on to the charismatic resident troublemaker, Lisa (Angelina Jolie), a sexualized sociopath who has been in and out of the institution for eight years. These young women not only become her closest friends, but light Susanna's way back to someone she had lost (herself).
Ultimately, Susanna must choose between the world of those who belong on the inside of the institution and the often difficult world of reality on the outside. Guided by no-nonsense ward nurse Valerie (Whoopi Goldberg) and the hospital's head psychiatrist, Dr. Wick (Vanessa Redgrave), she resolves to leave the "parallel universe" behind, reclaim her independence and continue life on her own with her own terms.