In Minneapolis, Minnesota, the self-absorbed Mavis Gary (Charlize Theron) is the divorced, 37-year-old ghost writer of the Waverly Prep series of young adult novels, who is on deadline with her editor to finish the last book of the soon-to-be-canceled series. While experiencing a creative block, Mavis receives an e-mail with pictures of the newborn daughter of her high school boyfriend Buddy Slade and his wife Beth Slade. Believing this to be a sign she and Buddy are meant to be together, Mavis returns to her hometown of Mercury, Minnesota, to reclaim her life with Buddy, under the pretext she is overseeing a real estate deal.
Upon arriving after multiple listens to Teenage Fanclub off a mixtape Buddy gave her in high school, Mavis invites him to meet her at a local sports bar for old times' sake the next day. In the interim, she goes alone to a different bar, Woody's. There she reconnects with a former classmate she barely remembers, Matt Freehauf, who became disabled after being beaten by jocks who erroneously assumed he was gay. Matt tells Mavis that her plans to destroy Buddy's marriage are insane, but Mavis ignores him.
The following day, Mavis meets Buddy at the sports bar, where they run into Matt, the bar's bookkeeper, who teases Mavis about her plans. On their way out, Buddy invites Mavis to a presentation of drummer Beth's "mom rock band", and asks her to autograph some of her books for Buddy's niece. Mavis agrees, and, after spending another night getting drunk with Matt, who distills homemade bourbon in the garage of the house he shares with his sister Sandra, attends the concert, where the other moms are resentful of what one remembers as the "psychotic prom queen bitch."
Buddy becomes uncomfortable with Mavis's advances and decides to leave. Beth wants to extend her girls' night out, however, so Mavis offers to drive the drunk Buddy home. On the lawn they share a kiss that's quickly broken up when the babysitter opens the front door to greet them. The next day, after an awkward encounter with her parents, Mavis is invited to Buddy's daughter's naming ceremony. She goes out with Matt again to Woody's, where they continue to bond and later bicker. The following day, Mavis attends the party, where she declares her love for Buddy, but he rebuffs her. Distraught, Mavis attempts to leave and runs into Beth, who accidentally spills wine on Mavis's dress. Mavis insults her, and tearfully reveals that she got pregnant with Buddy's baby years ago, but had a miscarriage after three months.
Buddy, who has been preparing a drum-set gift for Beth in the garage, opens the garage door and belatedly learns what's transpired. Mavis asks him why he invited her. He reveals it was Beth's idea, as she feels sorry for Mavis. Humiliated, Mavis leaves the party and visits Matt, where she breaks down in tears and initiates sex. The following morning, while Matt sleeps, Mavis has coffee in the kitchen with Sandra, who tells Mavis she's better than the rest of Mercury. Mavis realizes she needs to go back to the city. Sandra asks to go with her, but Mavis rebuffs her and leaves alone.
On her way out, she is able to write the last chapter of the book, in which the main character, much like Mavis, realizes the true meaning of being an adult, and both prepare to start their lives anew.