In a hopeful prologue, high school freshmen Annie, Jennette McCurdy, Marisa, Kiersey Clemons, and Kelly, Virginia Gardner, vow to open their college acceptance letters together at their senior bash.
Years later, Annie and Marisa remain best friends, but Kelly has stopped talking to both of them and has become the most popular girl in school. Annie is determined to keep the pact alive and open the letters at the senior party, but Marisa grows tired of Annie constantly talking about the idea. Kelly tries to reach out, but the other popular girls bully them and ruin her attempt at conversation.
In class, Annie spots Kelly and the other top girls laughing about something, sending her into a panic and driving her to the bathroom. Marisa goes to comfort her, but Kelly follows and pushes them outside, where they are caught by the annoying hall monitor, Karan Soni Andy, who assigns them detention. Marisa and Annie decide to skip the rest of the day and set off on little adventures to track down the location of the senior party.
At detention, Kelly sends Marisa a text, but Marisa deletes it, leaving Annie upset. After detention, Annie confronts Marisa, who loses her temper and yells at her to stop “living in a fantasy world” and give up on rekindling their friendship with Kelly; Annie breaks down in tears. The trio separately reminisces about how close they were during their freshman year, a memory that still tugs at them during moments of loneliness.
Annie sees Kelly and her popular friends getting into a limo to go to the senior party; she descends into despair and overeats in response to the ache of lost closeness. Marisa finds Annie moping by her car and reveals that the text from detention said that Kelly misses them. She hadn’t told Annie because she feared that if Kelly and Annie reconciled, Annie would abandon her. Annie accuses Marisa of running away from problems and not doing anything with her life. Marisa leaves to try to buy alcohol, hoping to dull the ache of unresolved friendships.
That night, Marisa encounters a familiar face at a liquor store: Sarah, Brianne Howey, one of the mean girls from their freshman year, now working a second job while pregnant. Sarah tells Marisa that life after high school gets “real,” and that she should just accept it as it comes. The encounter fuels Marisa’s spiraling night and she ends up drunk, wandering the quiet school as darkness falls.
Meanwhile, Kelly arrives at the senior party but seems less than thrilled, and she soon confronts Marisa, telling her she never wanted to end their friendship—it was Marisa who changed. Annie makes the choice to attend the party as well, gathering her courage and her crush, Phil, Moisés Arias, and they figure out where the party is by calling local liquor stores to learn which one is delivering alcohol to the venue and snag the address.
As they drive, a police siren sounds in the distance and they speed past it, triggering a tense police chase. In a adrenaline-fueled moment, they crash the car right into the party, shattering the line between conflict and reconciliation. The three girls finally reunite, embracing with a sense of relief and renewed connection.
In the quiet aftermath of the chaos, Annie, Marisa, and Kelly honor the promise they made four years earlier and open the letters together. The film ends on this moment of unity, leaving the contents of the letters unrevealed and inviting viewers to imagine what each of them might have hoped to find inside.
