Three weeks shy of her 28th birthday, Julianne "Jules" Potter (Julia Roberts), a food critic based in New York City, receives a life-altering call from her lifelong friend, Michael O'Neal (Dermot Mulroney). The two had made a pact years ago: if they were both single by the time they turned 28, they would tie the knot. However, Michael now shares the news that he will marry Kimmy Wallace (Cameron Diaz), a bright college student, within just four days! Kimmy is not only eight years younger than Michael but also the daughter of a well-off restaurant owner who happens to own the Chicago White Sox.
As the details of the imminent wedding unfold, we are introduced to Kimmy's family, including her mother, Isabelle Wallace (played by Susan Sullivan), and her supportive father, Joe O'Neal (M. Emmet Walsh), who is Michael’s father. The complexities grow with additional characters making an appearance, such as Kimmy's bridesmaids, the twin sisters Samantha (Rachel Griffiths) and Mandy Newhouse (Carrie Preston), and Scotty O'Neal (Christopher Masterson), Michael's younger brother.
Realizing that Michael is truly the love of her life, Jules takes a bold step by traveling to Chicago with the intent of wrecking his wedding plans. Once reunited with Michael, she is caught off-guard when Kimmy, full of trust and warmth, asks her to take on the role of maid of honor. Jules, caught in a web of her own making, attempts to sabotage the impending marriage.
Her plot thickens during a karaoke outing, where she tries to embarrass Kimmy by forcing her onto the stage. However, Kimmy surprises everyone with her vocal talent, winning over the crowd and proving Jules wrong. In a bold but morally questionable move, Jules even manipulates Kimmy into suggesting that her father offer Michael a job, fully aware this would stir trouble. The plan backfires spectacularly when Michael becomes upset but ultimately finds himself forgiving Kimmy during a vulnerable moment.
Feeling desperate and out of options, Jules enlists the help of her friend George Downes (Rupert Everett), who arrives in Chicago to advise her. Following George's suggestion, Jules concocts a scheme to make Michael jealous by feigning an engagement to George, who is, in fact, a gay man. Their lunch outing turns into an unexpectedly awkward spectacle when George shares an exaggerated romantic story, turning the spotlight on Jules in a rather embarrassing way. Despite this, Michael confesses to being jealous, providing Jules with a chance to lay bare her true feelings. Yet when the moment arrives, she hesitates, and they share a heartfelt dance to their shared song, "The Way You Look Tonight."
On the eve of the wedding, Jules plots again, utilizing Kimmy's father’s email to send a manipulative letter intended to get Michael fired so he can work for Kimmy's father instead. While the plan is meant to remain a secret, the email is unwittingly sent, leading to Michael's outrage and the cancellation of the wedding.
The following morning, Jules awakens to find out that Michael's and Kimmy's wedding plans are still, surprisingly, intact. In a last-ditch effort to break them apart, Jules finally confesses her love to Michael during a frantic chase. When Kimmy sees their passionate kiss, she escapes, prompting Michael to pursue her, with Jules in tow, determined to fix the chaos she has created.
Amidst the backdrop of Chicago's public places, Jules confronts Kimmy, who expresses her frustration and hurt over Jules' interference. Apologizing sincerely, Jules reassures Kimmy of Michael's genuine feelings for her, leading to a fragile truce.
As the wedding proceeds, Jules steps up as the maid of honor and shares a heartfelt toast at the reception, while both she and Michael realize the need to move on. They amicably designate "The Way You Look Tonight" as a temporary song for Kimmy and Michael until they create their own. In a surprising twist, Jules finds herself sharing a dance with George at the reception, where they celebrate newfound beginnings and farewells to past loves, marking a fresh chapter for all involved.
