Kate Fields, portrayed by Mamrie Hart, is a dental assistant feeling stuck and consistently disappointed by her love life, which consists of dull blind dates and lonely evenings. Her friend Evie Jones, played by Grace Helbig, is caught in a stifling marriage with Todd Jones (Andy Ridings), who drags her to philanthropic events that reflect his family's interests rather than her own. Meanwhile, Charlie, enacted by Hannah Hart, struggles in her desire to marry Claire (Kelen Coleman) while grappling with her own life issues, much to Claire's irritation. As Kate’s 30th birthday approaches, she finds herself despondent when her high school friends gather for Taco Tuesday and unveil letters they wrote to their future selves, encapsulating dreams of a successful career, a loving marriage, and children—goals that currently feel out of reach.
To lift Kate’s spirits, Charlie and Evie decide to throw her a surprise birthday bash, inviting old friends to attend at Evie's husband's parents' house, which happens to be empty that night. But there’s concern about Oliver (Murphy), a local teen who knows the elder Jones family and might let their plans slip. To prevent that, Evie extends an invitation to him and his friends, including his crush Kinsey (Hughes) and her brother Richard. Kate’s boss, Jules (Sertich), surprises everyone by joining the festivities with patient Dan (Dellapina), whom she's trying to set up with Kate. Just as merriment ensues, the atmosphere shifts with the unexpected arrival of Ashley, a controversial figure known for causing drama.
As Kate arrives, she finds herself engulfed in laughter and celebration, initially hesitant about the gathering’s size but soon swayed by the infectious energy of her friends. With the festivities in full swing, Kate connects with Dan in an easy-flowing conversation, even while Oliver feels out of place, longing for Kinsey, who arrives with her date Derrick (Leyva). Charlie dives into a series of risky bets against Peter (Chamberlain), directly undermining her promise to Claire. Furthermore, Evie has an awkward encounter with her ex-boyfriend Ben (See), but their interaction is cut short when Kate misinterprets Dan's departure, believing he left with Ashley.
The evening quickly takes a downturn for Charlie, who faces humiliation at Peter's hands, confirming Claire's skepticism about gambling just as she rushes out in embarrassment. During this chaos, Jules unexpectedly bonds with Richard, known as "Raven," who has shed his inhibitions. Meanwhile, Kate reminisces about her past relationship with Luke (Cole), seeking closure as he reminds her that it was ultimately her decision to end things.
The tone darkens as Evie uncovers the deep-rooted obsession Ben has for her, evident by her face tattooed onto his skin—this horrifying revelation prompts her to expel the rowdy group of teenagers from the house, given the escalating disorder. Amidst the final bursts of celebration, Kate, Evie, and Charlie stumble upon a broken champagne bottle left by Ashley, igniting a wild idea for revenge on their past antagonist.
As the trio enacts their plans, they are engulfed in an exhilarating spirit of celebration that leads them to Ashley's home, where toilet paper decorates the landscape. In a moment of vulnerability, Evie reveals her marital frustrations, contemplating divorce just as Ashley arrives with her secretive lover (Ahlquist). Chaos ensues when Ashley slips and injures her tooth, prompting Kate to step in and remove the tooth herself to prevent further issues, but their triumph is short-lived as police arrive to arrest them.
The night ends with the trio in jail, while Oliver escorts Kinsey home, where she realizes her relationship with Derrick isn’t what she truly wants. Just in time, Todd intervenes, getting the women released thanks to Dan’s initiative. With newfound clarity, Evie announces her intention to divorce Todd, an empowering moment in their narrative. Concurrently, Charlie reconciles with Claire, revealing that their earlier wager benefitted Peter’s business, while Kate and Dan clear up previous misunderstandings over breakfast, rekindling their bond.
In a heartwarming conclusion, Charlie and Claire’s engagement is celebrated, and Kate embraces the progress she has made in her profession and her budding relationship with Dan as well as her friendship with Evie, who finds comfort in her new living arrangement. The post-credits scene provides a haunting touch, revealing Ashley waking up horrified to discover she is missing a tooth, underscoring the night’s chaotic repercussions.
