College freshman Tracy Fishko (Lola Kirke) finds it challenging to navigate her new life at Barnard. Struggling with feelings of isolation, she eventually forms a friendship with a fellow student, Tony (Matthew Shear), and even develops a crush on him. However, when Tony starts dating another girl named Nicolette (Jasmine Cephas Jones), Tracy's feelings of loneliness resurface. Taking her mother's advice, she reaches out to her soon-to-be stepsister Brooke (Greta Gerwig), who lives in New York.
Captivated by Brooke's vibrant lifestyle, Tracy enjoys a whirlwind night of fun with her. Inspired, she writes a short story and submits it to a prestigious literary magazine at her college. As Tracy spends more time with Brooke, she learns about her ambitious plans for a quirky restaurant named "Mom's," a tribute to her late mother, which is partially funded by her partner. However, Brooke's life takes a turn when she returns home one night only to find herself locked out. To add to her distress, she discovers that her boyfriend has pulled his financial support, leaving her with mounting bills for the restaurant.
In a moment of desperation, Brooke seeks guidance from a psychic, accompanied by Tracy. The psychic's words lead Tracy to believe that Brooke should seek financial help from her estranged former friend Mamie-Claire (Heather Lind), who once stole Brooke’s business idea and married her ex-fiancé. Tracy persuades Tony to drive her, Brooke, and Nicolette to Mamie-Claire's Connecticut home, where they unexpectedly crash Mamie-Claire's book party. Hoping to secure an investment for her restaurant, Brooke decides to pitch her idea, while Tracy supports her nervously.
When Mamie-Claire's husband Dylan (Michael Chernus) arrives, he surprisingly encourages Brooke to present her concept. Initially stammering, Brooke gains confidence with Tracy’s assistance. Eventually, Dylan offers to give Brooke the funds, but he clarifies that it would strictly be for paying off her restaurant debts, not an investment. Disheartened by her former friend's discomfort, Brooke ultimately declines the offer.
Tensions rise when Nicolette, suspicious of Tracy and Tony's relationship, confronts Tracy about her story, which becomes the talk of the party. Brooke, feeling betrayed by how Tracy described her, tells her that their hopes of sisterhood are dashed because their parents no longer intend to marry, and she threatens to sue. Devastated, Tracy returns home.
Despite her acceptance into the university's literary society, Tracy struggles to find a sense of belonging and decides to establish her own literary club, inviting both Tony and Nicolette to submit their works. As Thanksgiving approaches, finding herself alone, Tracy decides to visit Brooke’s old apartment, where she discovers Brooke packing up for a move to Los Angeles. Brooke reveals that Mamie-Claire helped her settle her debts by giving her the proceeds that could have come from the stolen t-shirt business. Excitedly, Brooke shares that she passed her SATs and has been accepted into college.
In a touching final gesture, Tracy invites Brooke to celebrate Thanksgiving together, and the two share a heartfelt meal at a restaurant, cherishing their friendship one last time.
