Toula (Nia Vardalos) is going through an early midlife crisis. At thirty, she is the only woman in her family who has "failed". Her family expects her to "marry a Greek, make Greek babies, and feed everyone until the day [she] die[s]." Instead, Toula is stuck working in the family business, a restaurant. In contrast to her "perfect" sister, Athena (Stavroula Logothetis), Toula is frumpy and cynical. She fears she's doomed to be stuck with her life as it is.
At the restaurant, she briefly sees Ian Miller (John Corbett), a handsome school teacher. This event, combined with an argument with her father, Gus (Michael Constantine), motivates her to begin taking computer classes at a local college. She also gets contact lenses, wears her hair curly, and begins to use makeup. Toula, her mother (Laine Kazan), and her aunt Voula (Andrea Martin) then contrive a way to get Gus to allow her to work at Voula's travel agency.
Toula feels much better in her new job, especially when she notices Ian hanging around looking at her through the window. They finally introduce themselves and begin dating. Toula keeps the relationship secret from her family until some weeks later when Gus finds out. He throws a fit because Ian is not Greek, but Ian and Toula continue to see each other against Gus's wishes. Ian proposes marriage to her, she accepts, and he agrees to be baptized in the Greek Orthodox Church to be worthy of her family.
As the year passes, the wedding planning hits snag after snag as Toula's relatives "helpfully" interfere. Her father insists on inviting the entire church to the ceremony, her mother orders the invitations but misspells Ian's parents' names, and Toula's cousin Nikki (Gia Carides) orders tacky bridesmaids' dresses. Toula is horrified to learn that her parents invited the entire family to what was meant to be a "quiet" dinner, and the Millers, not used to such cultural fervor, are overwhelmed.
The wedding day dawns with liveliness and hysteria, but the traditional wedding itself goes without a hitch. Gus gives a speech accepting Ian and the Millers as family and buys the newlyweds a house right next door to him. The film's epilogue shows the new couple's life six years later in which they have a daughter, Paris, whom they raise in the Greek style, but Toula tells her she can marry anyone she wants when she grows up after she says she wants to go to Brownies instead of Greek school.