Jimmy Callahan provides best man services, through The Best Man Inc., for guys who don't have the friends needed for a wedding. Doug Harris, a successful tax attorney, and his fiancée Gretchen Palmer are planning their wedding. Frantically needing a best man and groomsmen, Doug contacts Jimmy's company.
Doug needs a "Golden Tux" (seven groomsmen) to match with Gretchen's bridesmaids, which has never been done before. Jimmy agrees to do it for $50,000, all expenses paid.
Three of Jimmy's friends are recruited as groomsmen: Fitzgibbons, a federal prison escapee, Lurch, who has a nagging wife and Reggie, an airport security guard. They interview for the four remaining spots: they choose Kip, sexy with a stutter; Endo, has three testicles; Bronstein, can dislocate and relocate his shoulder; and Otis, can say sentences backwards.
Doug tells Gretchen that "Bic" is a military priest based in El Salvador, and she insists that he also come to the family brunch. There, Doug gets nervous, almost blowing his cover until Jimmy accidentally sets Gretchen's grandmother on fire. In the emergency room, Jimmy lies to Gretchen's father Ed, that they used to play football. So he challenges them to a match with some of his old college teammates.
Doug meets his groomsmen, all with fake identities based on the last names of famous Los Angeles sports figures—Plunkett, Rambis, Garvey, Alzado, Drysdale, Carew, and Dickerson. Jimmy takes Doug and the groomsmen on fake photo shoots of skydiving, scuba-diving, etc..
When Doug begins to have doubts, they visit Edmundo, who tells him the key is to please Gretchen and her mother Lois and nothing else. To prove how good he is at being a best man, Jimmy takes Doug to a wedding where the best man makes a terrible speech. After, they have drinks and show off their dance moves. Jimmy tells him how he gave an excellent best man speech for an acquaintance which led to his career as a wedding ringer. Doug explains that he moved a lot with his parents, so he never got to make friends. When his parents died, Doug took over the business, and work consumed him, leaving him without friends and therefore a best man. Jimmy drives Doug home, and reiterates that they are in a business relationship, and Doug, although hurt, agrees.
At Doug's outrageous bachelor party he is introduced to Nadia, who tries to seduce him, but Doug instead befriends her. A prank involving peanut butter, a dog and a blindfolded Doug's goes awry, requiring the groomsmen and Nadia to rush him to a hospital. When he wakes up the next day, Nadia kisses him goodbye, she hints she would like to know him better. Later, the groomsmen play football with Ed and his college football friends, including Joe Namath, John Riggins, and Ed "Too Tall" Jones. A mud bowl ensues, Ed blows out his knee on the last play and the wedding party team wins.
At the rehearsal dinner, Gretchen's bridesmaids sing a song, while Doug's groomsmen create a slideshow of the fake pictures they took, winning Gretchen over. That night, she notices the Bic razors and Mitchum deodorant in the cabinet, and recognizes the last names of the groomsmen and deduces the scheme. Asking Doug about it, he brushes it off, saying she is paranoid, to which she reluctantly agrees.
On the wedding day, the family priest cancels, so Doug suggests that Jimmy officiate. At the reception, Jimmy congratulates Gretchen, who exclaims that the wedding is a disaster as her zipper is torn, her grandmother has third degree burns, her dad's knee is blown out, the food is bad, and she doesn't love Doug (saying she only married him because he is nice and can easily afford her lavish lifestyle). Doug overhears, telling Jimmy that he can't go through with it, but Jimmy dismisses this.
As Jimmy gives his best man speech, Doug stops him, revealing they aren't really married as "Bic" is not a real priest, nor are his groomsmen who they claim to be. Gretchen is livid that her wedding is ruined. Ed's college friends make peace with Doug and Jimmy and tell them they were good players. Jimmy gets a date with Gretchen's sister, Allison. As they leave, Doug pays Jimmy his $50,000 fee and they accept each other's friendship. Jimmy then has the idea to cash in Doug's first class honeymoon tickets to Tahiti, going instead on a guys trip, along with Nadia (who is now with Doug).
