Annie Walker, portrayed by Kristen Wiig, is a single woman in her mid-30s, navigating life in Milwaukee, Wisconsin. After experiencing the downfall of her bakery, Cake Baby, she's left without a boyfriend, savings, or much direction, finding herself working in a jewelry store. Her eccentric artist mother (Jill Clayburgh) suggests she moves back home, but Annie prefers her chaotic living situation with British immigrant Gil (Matt Lucas) and his uninvited sister, Brynn (Rebel Wilson), who intrusively reads Annie's personal journal. Although she engages in a casual relationship with the self-absorbed Ted (Jon Hamm), she yearns for something more meaningful.
Annie's closest friend, Lillian Donovan, played by Maya Rudolph, announces her engagement to a wealthy banker from Chicago and asks Annie to be her maid of honor. During the engagement party, Annie is introduced to the other bridesmaids, including Lillian's cynical cousin Rita (Wendi McLendon-Covey), the idealistic Becca (Ellie Kemper), the boisterous future sister-in-law Megan (Melissa McCarthy), and the glamorous but overly ambitious Helen Harris III, portrayed by Rose Byrne. Instantly, a rivalry sparks between Annie and Helen as both women vie for Lillian's attention.
The tension escalates as they participate in a game of doubles tennis, where their competitiveness turns physical, each intentionally hitting the other with volleys. Following this, Annie plans a lunch at a Brazilian steakhouse for the bridal party. While Lillian steps away, Annie proposes a Paris-themed bridal shower, but Helen dismisses the idea, showcasing her dominant personality. At a bridal boutique, Helen exploits her connections to secure a visit, as Annie inadvertently overlooks the need for reservations. Soon after, the group suffers food poisoning, orchestrated by Helen's ordering choices, highlighting her knack for one-upping Annie.
Annie, aware of her tight finances, suggests having the hens party at Lillian's parents' beach house, yet Helen insists on a lavish trip to Las Vegas. Out of pride, Annie insists on sitting in coach while Helen takes first class. On the flight, Annie's fear of flying causes a series of mishaps, forcing an emergency landing, which leads to the cancellation of the hens party. On the bus ride back, Annie's attempt to apologize is met with rejection as Lillian leans towards Helen for wedding planning.
Despite her hopes of rekindling things with Ted, Annie starts to form a connection with Officer Nathan Rhodes (Chris O'Dowd), a friendly traffic cop who had previously let her off with a warning. While Nathan motivates her to dive back into baking, Annie's past trauma from her bakery's failure envelops her, pushing her away from the idea. After a night spent together, their relationship strains as Annie reacts to his offer of help with resentment, leading to her sudden dismissal from her job after a public outburst.
The bridal shower, curated by Helen, ends in disaster as Annie's emotional outburst leads Lillian to ask her not to attend the wedding. Annie's life spirals as she isolates herself, only to be confronted by Megan, who shares her inspiring journey of overcoming adversity. This pivotal moment helps Annie realize her mistakes, leading her to attempt an apology to Nathan, making him a cake in his honor—though it ultimately ends up as wildlife food.
On the wedding day, with reluctance, Annie is drawn back into the festivities when Helen seeks her aid in finding the missing Lillian. Amidst a heartfelt journey alongside Nathan, they locate Lillian at her apartment. Together, Annie supports Lillian in preparing for her big day.
Taking up her role as maid of honor once more, Annie witnesses the extravagant wedding organized by Helen, featuring neon signs, fireworks, and even a special performance by Wilson Phillips. In a surprising twist, Helen extends an olive branch after the ceremony, expressing a desire for friendship. Noticing the chemistry brewing between Annie and Nathan, Helen orchestrates a chance for them to leave together in his squad car, sirens blaring in celebration. In a humorous post-credits scene, Megan is caught filming a steamy encounter with an air marshal, adding a final touch of hilarity to the adventure.
