In 1984, Catherine "Cat" Storm is a teenager attending an elite preparatory school in Manhattan. Her mother seems strict and cold, her father is absent from her life, and her little brother is a hypochondriac. Her best friend is Delilah Milford, a free-spirited delinquent whose father is a major stockholder in ShopRite.
After Delilah is expelled from school for drug possession, Cat befriends Grace Bailey, an English exchange student who helps her get in with the popular crowd. The pair begin to attend high-brow parties thrown by queen bee Peg, where a gay ephebophile named Kenny supplies the attending teenagers with cocaine. Cat doesn't participate in her friends' drug use, which many of them look down on her for, but she is defended by William Sellers, a childhood friend from dance class. She quickly grows infatuated with William for seeming so sensitive and attentive to her. Meanwhile, Delilah grows resentful of Cat for abandoning her, sending her dead flowers on her birthday.
While partying, Cat's new friends gossip about the string of jewellery thefts happening in their apartments. They also find out Delilah's father has been arrested for kidnapping, after which they mock the Milford family's dysfunctions. Cat and William skip school for a walk in the park, where William opens up about his addiction. Later, Cat tells her mother that William has asked her out to Peg's upcoming Christmas party. When William arrives to pick Cat up, he watches from the entryway as the pair argue, with Ms. Storm harshly expressing her awareness and disapproval of the boy's delinquency. Cat insists he's a good person and storms off to the party.
Peg's party is crashed by Delilah, who confronts Cat in the bathroom before defecating in the group's ice bucket and leaving. William pressures Cat into taking drugs, and feeling suffocated by her difficult home life, she goes overboard and experiences a blackout. The pair have sex, and an inebriated Cat confesses her last name is a pseudonym and her father is really Jewish.
The next day, Cat returns to her worried sick mother, who accuses her of borrowing an expensive pair of earrings without permission. This leads to another argument, and Cat is quick to storm out once more. She tries to find William, getting a glimpse of his abusive, impoverished household in the process. They meet at a local recreation center, where William is suddenly acting cruel towards Cat for seemingly no reason. While trying to blow her off, an expensive watch falls from his hoodie pocket, and Cat realizes he's the one stealing jewellery from everyone's apartments, including her mother's earrings.
Cat tries to tell Grace that William is the thief, only to get the cold shoulder from her. All of Cat's friends start ignoring her at school, and she hears from a peer named Eloise that William has been telling everyone about their intercourse at the Christmas party. Cat tries to confront Grace, who reveals that William also told everyone about her secret Jewish lineage, which makes Grace no longer want to associate with her. Suddenly ostracized, Cat starts trying to reconnect with Delilah, but can't find her anywhere.