"Girl, Interrupted" is Sigmund Freud's idea of a slumber party: a cuckoo's nest full of noisy young chicks who stay up late, raid their psychiatrist's office and steal a peek at their diagnoses. Giggles, then silence.

Based on Susanna Kaysen's pre-Prozac memoir, this affecting if familiar fare chronicles the author's two-year stay at Claymoore, a chichi psychiatric hospital outside Boston. Susanna (Winona Ryder), like lots of other teenagers, is dazed and confused and something of a pill. But after a hapless suicide attempt and a perfunctory examination, she's diagnosed with Borderline Personality Disorder and winds up in "the bin."

It's probably the best thing that ever happened to this poor little rich girl, who isn't like her classmates, all of whom are bound for Harvard, Wellesley or Princeton. She's on a borderline, all right, but not between sanity and insanity. Essentially, the 18-year-old can't quite bring herself to leave girlhood behind, and "Girl, Interrupted" is about her fight to come of age.

The story is set in the late '60s--Vietnam footage is regularly interspersed with the action--and Susanna's plight mirrors that of the shy, aimless Benjamin Braddock in "The Graduate." Like him she sleeps with a married friend of her parents' and is contemptuous of her family's values yet incapable of formulating her own. At Claymoore, she initially nurses her blue mood, even relishes the cachet of being among the fashionably depraved.

Ryder turns in a cool, graceful performance, but acting honors go to Angelina Jolie, splendidly menacing as Lisa, a heartless, wild-haired sociopath who plays the id to Susanna's superego. Together they make a whole personality, albeit an unstable one, and in time Susanna realizes that her rebellious best friend is hardly worth emulating. Maybe she'd better think about getting on with the rest of her life.

The stellar young supporting cast includes Clea Duvall as Susanna's "Wizard of Oz"-obsessed roommate, Brittany Murphy as a pampered rich girl who eats nothing but rotisserie chicken, and Elisabeth Moss as an endearingly childlike burn victim. Vanessa Redgrave, Whoopi Goldberg and Jeffrey Tambor have less intriguing roles as the girls' long-suffering caretakers.

The drama is slight, but director and co-writer James Mangold ("Cop Land") keeps the pace from faltering and the energy from ebbing despite the story's tired conventions, hoary themes and obvious conclusions. Like many films about mental illness, "Girl, Interrupted" reminds us once again that the world is full of crazy people, most of whom aren't in asylums. They're on Capitol Hill starting wars.

We've definitely been on this ward before.

Girl, Interrupted (125 minutes, at area theaters) is rated R for profanity, sexuality and drug use.

CAPTION: Winona Ryder, right, and Angelina Jolie as two unbalanced young women.

CAPTION: Winona Ryder turns in a graceful performance in "Girl, Interrupted."