Paul Morse, portrayed by Jason Lee, is on the brink of tying the knot with Karen Cooper, played by Selma Blair, in Seattle. As the bachelor party unfolds, Paul finds himself engaging in a deep conversation with a dancer named Becky Jackson, beautifully represented by Julia Stiles. Their chemistry is undeniable, but the following morning brings a shocking surprise when Paul wakes up to find Becky beside him. Fearing the worst and assuming they had spent the night together, he hastily sends Becky away, desperately hoping to avoid any future interactions.
In the days leading up to the wedding, he attempts to suppress the incident while preparing for Karen's arrival at his apartment. To conceal his tracks, Paul resorts to an absurd tactic of hiding Becky's underwear in the toilet tank. However, fate has different plans when Becky reappears in his life as the toll booth collector and, to his dismay, discovers that she is actually Karen's cousin.
Meanwhile, tensions rise as Ken, Karen's father, feels slighted by not being invited to the bachelor party—complicating Paul’s already precarious situation since he is employed at Ken's marketing firm. As if things couldn't get worse, Paul finds himself dealing with an uncomfortable case of crabs, leading him to a pharmacy where he encounters Sandra, Karen's mother. In a moment of panic, pharmacist Jeff, portrayed by Fred Ewanuick, fabricates a story about being a chef who has cooked crabs for Paul, landing him a gig to cater the rehearsal dinner.
During a chaotic family dinner, Paul’s attempts to stay incognito fail spectacularly when he accidentally triggers a car alarm while trying to hide from Karen’s family outside. Ultimately, he ends up having an awkward introduction with Becky. The next day, after tracking her down at a music store, Becky reassures him that they didn’t have sex, which alleviates some of his worries. However, complications arise when her vindictive ex-boyfriend, Ray Donovan, instigates trouble for Paul by revealing that he has been following Becky.
In the midst of all the chaos, Karen finds the hidden underwear, causing a seismic shift in Paul's attempts to maintain his facade, leading to humorous and tense confrontations. Minister Ferris, Paul's neighbor, also adds to the mix of mayhem, threatening Paul's reputation with suggested misconduct. As the narrative unfolds, hints of Pete's affection for Karen resurface, making for an even more tangled web of relationships.
After a series of misadventures that include sneaking into Ray's apartment and getting caught up in police involvement due to Ray's schemes, Paul begins to recognize a thrilling side of himself that only emerges when he is around Becky. The wedding day looms, and in a moment of overwhelming anxiety, Paul confides in Pete about the whirlwind of events leading up to the nuptials. Ken's words of encouragement push Paul to confront his feelings.
During the ceremony, the unexpected occurs when Minister Ferris expresses his disapproval, prompting Paul to stand and voice his doubts about marrying Karen. In a shocking turn of events, Pete's heartfelt proposal to Karen resonates, leading her to accept. Consequently, Paul seizes the moment and pursues a relationship with Becky, who brings out his adventurous spirit and awakens a side of him he never knew existed.
