Massie Block is the twelve-year-old leader of the Pretty Committee, a clique of popular, wealthy students at her upstate New York all-girls private school, Octavian County Day. Her father informs her that Jay Lyons, a friend from Florida, will move into the Block family's guest house with his wife and two children until they can find a permanent residence. Massie is horrified that the unexpected guests are disrupting her plans and occupying her space, but her parents insist she try to get along with Jay's daughter Claire Lyons, who is Massie's age and will be attending her school.
Anxious for the approval of her wealthy peers, Claire convinces her mother to take her shopping. At the mall, she encounters Dylan Marvil and her friends Alicia Rivera and Kristen Gregory. Meanwhile, Massie encounters an older boy named Chris Abeley and is immediately smitten with him. He makes plans to go riding with her again, which she interprets as a romantic gesture.
The next day, Claire accompanies Massie on her ride to school. On the way, they pick up Dylan, Alicia, and Kristen; despite having been friendly to Claire the day before, the girls grow cold and dismissive towards her upon realizing Massie dislikes her. Once at school, Claire encounters Chris and also finds herself attracted to him. Witnessing the interaction, Massie is overcome with jealousy, and enlists the other members of her clique to start bullying Claire via harsh insults and cruel pranks.
Rejected by the so called "Pretty Committee", Claire decides to sit with someone else at lunch and befriends Layne Abeley, Layne Abeley Chris' eccentric younger sister. The two make plans, but Claire later abandons them to attend the Pretty Committee's weekend sleepover, which Massie's mom has invited her to. The girls continue to antagonize Claire at the sleepover, resulting in Claire leaving early. Massie later starts acting friendly towards Layne with the intent of obtaining more information about Chris.
One night, Claire visits Massie's room to confront her, but the latter is gone. Seeking revenge, Claire logs onto Massie's IM account and messages the other three girls pretending to be Massie; first tricking Alicia into dressing like Claire, then body shaming Dylan, and finally getting Kristen to confide that she is secretly poor and attends OCD on a scholarship, which is why she's constantly stressed about maintaining top grades. One by one, the girls start to dislike Massie and decide to kick her out of the Pretty Committee, welcoming Claire into the clique instead.
After Kristen confronts Massie about the messages, the two deduce that Claire was the one behind them. The bullying starts anew, now greater in severity. Claire eventually breaks down in front of her mother, who encourages her to remain true to herself. Massie decides to surprise Chris by jumping out of a pop out cake at her family's upcoming charity auction while wearing a shirt that reads "I Love Chris Abeley." Layne, having realized that Massie was just using her, makes amends with Claire.
On a field trip to New York City, the Pretty Committee tries to sell a makeup line created by Kristen for a school project, but their recipe contains peanut oil, triggering allergic reactions in their classmates. Amidst the panic, Claire anonymously texts Layne's oatmeal to reduce the swelling. This proves successful, and all the other students regard Claire as a hero.
At the charity auction, Claire reaches out to the other members of the clique, who all accept her apology and commend her for her efforts to stand up to Massie, revealing that they secretly harbor resentment for Massie's cruel and controlling nature as well. After learning that Chris has a girlfriend, Claire prevents Massie from jumping out of the cake to save her from embarrassment. Massie thanks her, and admits that she targeted Claire because she feared her friends would pick Claire over her otherwise. That night, Massie types a note on her computer that Claire is now welcome in the Pretty Committee, smiling to herself.
