Party animal Rick Gassko (Tom Hanks), who makes his living as a Catholic-school bus driver, decides to settle down and marry his girlfriend Debbie Thompson (Tawny Kitaen). After learning the news of the engagement, Rick's shocked friends, led by Jay (Adrian Zmed), decide to throw him an epic bachelor party. The bride's wealthy, conservative parents are unhappy with her decision, and her father enlists the help of Debbie's ex-boyfriend Cole Whittier (Robert Prescott) to sabotage her relationship with Rick and win her back.
While Debbie worries and goes off to a bridal shower thrown by her friends, Rick heads to the bachelor party, which takes place in a lavish, spacious hotel suite, and promises to remain faithful. Both parties start off on the wrong foot because of Cole's meddling. As the bachelor party starts to heat up, Debbie and the girls decide to get even with Rick and his friends by having a party of their own. Both parties eventually collide, leading to Debbie accusing Rick of infidelity.
The bachelor party becomes a wild, drunken orgy and the hotel room is trashed, which infuriates the hotel's frustrated manager (Kenneth Kimmins). Adding to the confusion is Rick's friend Brad, who has become despondent over the breakup of his marriage and botches several suicide attempts. When Brad tries to slit his wrists with an electric razor, Rick says, "Well.....at least your wrists will be smooth and kissable."
Rick convinces Debbie of his love and faithfulness just as the party is raided by the police. In the ensuing melee, Rick and Debbie become separated and Cole kidnaps Debbie, so Rick and his friends chase after them. The chase culminates in a showdown between Rick and Cole in a 36-screen movie theater, with a fist fight taking place in synchronization with a similar fight being shown in a 3D film projected behind them; the audience believes that the real fight is an extraordinary 3D effect. Rick wins the fight and is reunited with Debbie.
After the wedding, Rick and Debbie are driven to the airport for their honeymoon in Rick's school bus, which is driven by a laughing Brad.
