Plot
In 2002 in Ridgefield, California, awkward teen Marni Olsen is bullied, especially by popular girl J.J. Marni's protective brother, Will, is a basketball star, oblivious to J.J.'s behavior, even when J.J. sabotages the team's crucial win by pushing the mascot-clad Marni into him.
Eight years later, Marni is a successful public relations executive in Los Angeles, recently promoted to their New York City office. She returns home for Will's upcoming wedding to a woman named Joanna, whom she has never met, and later learns is J.J.
To Marni's chagrin, her family adores Joanna, and Joanna claims to not remember her. Marni and Will's mother Gail unhappily discovers that Joanna's beloved aunt Ramona, a wealthy businesswoman paying for the wedding, is her former high school best friend.
Marni refrains from telling Will about Joanna's past. A series of mishaps have Marni resembling her high school self, further distressing her. Marni realizes that Joanna does remember her when Joanna deliberately plays the song she used to torment Marnie with in high school. Resolving to save Will from Joanna, Marni retrieves a video from the time capsule she buried during senior year which showcases Joanna's bullying. Meanwhile, Gail and Ramona reconcile, though Ramona still seems unhappy with her.
Joanna's ex-boyfriend Tim is devastated to learn that she is marrying Will. At the bridal shower, Marni privately confronts Joanna, who finally admits she remembers her, but warns her to not interfere with her and Will's relationship rather than apologizing.
Now more determined than ever to protect Will, Marni invites Tim as her date to the rehearsal dinner, hoping to cause problems. During toasts, Tim embarrasses Joanna by announcing she left him at the altar on their wedding day. Later, the time capsule video is played, shocking the guests. An especially confused and upset Will leaves.
Marni and Joanna fight, during which Marni insists Joanna hasn't changed, though Joanna remorsefully insists she had pretended to forget Marni so they could start over. Marni eventually destroys one of the wedding gifts, and Joanna retaliating by dump a bowl of cold soup on her head. Will scolds them both, before storming out and calling off the wedding. Ramona then calls Gail out for overshadowing her in high school and stealing her crush, Richie Phillips. During their argument, they fall into the pool, but ultimately reconcile. Gail's husband and Marni's father, Mark, grounds both Marni and Gail.
That night, Marni finds Joanna tearfully binge eating in her wedding dress. Joanna finally apologizes for bullying Marni, insisting that she changed after her parents died as she hoped to become someone they could be proud of, and explaining that she pretended not to know Marni because she feared her past would cost her Will and his family. Marni forgives her, promising to mend Joanna's relationship with Will. Marni later apologizes to Will, insisting she was only trying to protect him.
Joanna and Will reconcile in the family's old tree house, but because their younger brother Ben had loosened the screws, it collapses, injuring them and forcing a hospital stay which delays the wedding. However, Marni arranges a wedding at the hospital. Gail surprises Ramona with Richie, and they start a relationship while Marni starts one with Charlie, her brother's best friend.
Joanna reintroduces her grandmother, Bunny to Bunny's high school rival, Helen. Bunny finally gets revenge by stealing Helen's dance partner, while Helen swears that the game is not yet over.
