In a nostalgic flashback captured on a video camera, we are transported to New Year's Eve in the early 1990s, where five high school friends revel in their youth, sneaking into a hotel pool for a spontaneous and carefree skinny-dipping session. Fast forward a decade, and we meet Megan, played by Keira Knightley, who is now a 28-year-old attractive woman living in Seattle. Despite her advanced degree from a prestigious college, she finds herself working as a sign girl in her father's tax advice business, much to his delight, though he seems to enable her lack of ambition.
Megan is in a relationship with Anthony, portrayed by Mark Webber, a budding photographer. As the story unfolds, Megan reconnects with her close friends, Allison (Ellie Kemper), Savannah (Sara Coates), and Danielle (Kirsten deLohr Helland), at a gathering for Allison's upcoming wedding. This reunion is marked by uncomfortable moments, like when Megan jokes about a Buddha statue in a bridal shower game, which leads to tension among the friends.
During a wedding attended with Anthony, he drops to one knee to propose, but Megan panics, claiming it's not the right moment. After a series of humorous and cringe-worthy events, including accidentally discovering her father's romantic involvement with Allison's mother, Megan struggles with her feelings of inadequacy and her desire for freedom from expectations.
In a chance encounter at a supermarket, Megan meets a teenage girl named Annika, played by Chloë Grace Moretz, who enlists her help to buy alcohol for her and her friends. This impulsive act throws Megan into a whirlwind of chaotic teenage adventures, from toilet papering houses to drinking in the park while reflecting on the awkward moments she shares with her friends.
Megan’s escapades lead her to pretend to be Annika's mother at a parent-teacher conference, a decision that spirals into a week-long social experiment where she relives her youth. As she spends more time with Annika and her friends, she grapples with her own life choices and the complexities of adult relationships.
The boundaries between their lives blur further when Megan finds herself involved with Annika's father, Craig, played by Sam Rockwell. Their relationship blossoms unexpectedly, but it also complicates Megan's situation when Annika discovers their secret. Amid this turmoil, misunderstandings and confrontations arise as the group navigates their emotions leading up to prom night, a pivotal event that serves as a culmination of their journeys.
Megan's internal struggle crescendos when she realizes she can't continue this charade and must confront her true feelings regarding her past and present. The prom, set on a boat, becomes a turning point for Megan as she witnesses Annika take a leap of faith in pursuing her own happiness. Ultimately, Megan decides to embrace honesty, choosing to leave the past behind, acknowledging her love for Craig, and hoping for a chance to rekindle what they shared as she leaves behind a heartfelt invitation for another opportunity.
