In 1984, Cat Storm is a teenager attending an elite preparatory school in Manhattan. Her mother, Lily Storm, seems strict and cold, her divorced father is largely absent, and her younger brother is a hypochondriac. Her best friend is Delilah Milford, a free-spirited delinquent whose father is a major stockholder in ShopRite.
After Delilah's expulsion 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 hosted by Peg, where a gay ephebophile named Kenny supplies cocaine. Cat doesn't participate in the drug use, but she is defended by William Sellers, a childhood friend from dance class. She quickly grows infatuated with him 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 learn 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. This leads to another argument, and Cat storms out once more. She tries to find William, getting a glimpse at his abusive, impoverished household in the process. They meet at a local recreation center, where William is suddenly acting cruel toward 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, and she hears from a peer named Eloise Logan 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. Ostracized, Cat starts trying to reconnect with Delilah, but can't find her anywhere.
Still avoiding her mother, Cat falls asleep in the lobby of Delilah's building. She's awoken by Eloise, who's going to The Hamptons with her family. Cat tags along after hearing Delilah will be at a party in the countryside. She successfully finds an inebriated Delilah at the beach party and apologizes for abandoning her. The two make amends, and Delilah walks to a nearby gay bar to ask Kenny for a ride back to the city. There, she witnesses Kenny pocketing all the stolen jewellery that William has sold to him. While leaving, she stumbles upon William receiving fellatio from a male drug dealer. Fretting over how much Delilah has just seen, William chases her into the woods and tries to explain himself. Things escalate until he pushes her, causing her to hit her head on a rock. Delilah screams at him and threatens to press charges, and in a panic, he beats her to death.
After Delilah's sudden disappearance, Cat is forced to return to Eloise's house alone. Delilah's corpse is found the next morning, and William is swiftly arrested. Reporters interview the students with questions about Delilah, and they all deem her "reckless" and have little to say of her. Grace and Peg continue to defend William following his incarceration. Devastated and grieving, Cat makes amends with her mother, finally realizing her family is there for her if no one else is.
