At dinner, Jim Levenstein prepares to ask Michelle to marry him when Jim's dad calls to inform him that he has the ring. Michelle misinterprets when Jim stalls the question and Jim's dad arrives as he is receiving fellatio from under the restaurant table. The mishap grasps the restaurant's attention but Jim proposes and Michelle accepts.
Jim wishes to exclude Steve Stifler from the wedding; who becomes upset when he learns of this. Stifler agrees to teach Jim how to dance if he is allowed at the wedding. Jim asks Stifler to tone down his obnoxious personality in exchange for planning the bachelor party. Jim, Stifler, Paul Finch, and Kevin Meyers travel to Chicago when they discover there is only one designer who makes the dress that Michelle wants. Stifler unwittingly walks into a gay bar, and his raucous behavior gets him into a dispute with several patrons. Stifler defeats Bear in a dance-off, who offers to provide strippers for the bachelor party. Dress designer Leslie reveals himself and agrees to make the dress for Michelle. Michelle's parents, who were initially skeptical of Jim, agree to have dinner with him.
Michelle's younger sister, Cadence, flies in for the wedding. Both Finch and Stifler are attracted to her, and in an effort to win her over, they each adopt the other's personality. Stifler arranges the bachelor party but does not inform Jim, who unknowingly has arranged dinner with Michelle's parents at his home. With assistance from Bear, who poses as a butler named "Mr. Belvedere", Jim nearly succeeds in keeping the activities a secret, until Michelle's mother opens a closet door and is shocked to find Kevin inside; blindfolded, stripped to his boxers, and tied to a chair. The boys explain that it was an failed attempt to make Jim seem like a hero, and Michelle's parents tell him that if he puts that much effort into the upcoming marriage, they can give their blessing.
Michelle is concerned that Jim's paternal grandmother disapproves of the wedding because she is not Jewish. On the night before the wedding, Stifler inadvertently disrupts the walk-in refrigerator's power supply while retrieving a bottle of champagne in an attempt to seduce Cadence which kills the flowers. Previously, Stifler—unaware of Cadence's presence—had revealed his true rude and obnoxious personality. Angry, Jim asks him to leave, and all the others, including Cadence, support Jim's decision.
Feeling guilty for his thoughtless behavior, Stifler convinces the florist to put together a new batch of flowers, and he enlists the help of his football players and Bear. As a gesture of remorse, he also gives a rose to Cadence. Moved by his actions, Cadence agrees to have sex with him in a supply closet before the ceremony, but Stifler's presence is delayed by a brief thank-you meeting Jim calls among his groomsmen, citing how he is grateful to have friends like them. Quickly returning to the hotel, Stifler hears someone in the supply closet and steps inside, unaware that Cadence was interrupted by wedding preparations and that the ushers, the "MILF guys" from high school, placed Jim's grandmother in that closet to sustain her hostility about the wedding; Stifler only realizes this upon walk-in by Finch and Kevin. She becomes pleasant, particularly towards Stifler, making Michelle and Jim's dad unknowingly happy.
Despite the chaotic events leading up to it, Michelle and Jim get married. At the reception, the couple dances while Stifler dances with Cadence. Finch sits by himself when Stifler's mom arrives. Although agreeing they are over each other, Stifler's mom mentions having a hotel suite and invites Finch to join her. The film ends with the "MILF guys" spying on Stifler's mom and Finch having sex in the hot tub.
Jason Biggs as Jim Levenstein
Seann William Scott as Steve Stifler
Alyson Hannigan as Michelle Flaherty
Eddie Kaye Thomas as Paul Finch
Thomas Ian Nicholas as Kevin Myers
January Jones as Cadence Flaherty
Eugene Levy as Jim's Dad
Molly Cheek as Jim's Mom
Deborah Rush as Mary Flaherty
Fred Willard as Harold Flaherty
Angela Paton as Grandma
Eric Allan Kramer as Bear
Amanda Swisten as Fraulein Brandi
Nikki Schieler Ziering as Officer Krystal
Lawrence Pressman as Head Coach
Reynaldo Gallegos as Leslie Summers
Loren Lester as Celebrant
Justin Isfeld as Justin
John Cho as John
Jennifer Coolidge as Stifler's Mom
Julie Payne as Mrs. Zyskowski
Willam Belli as Butch Queen
David St. James as Front Desk Manager
known as American Wedding or American Pie: The Wedding, in some countries
sequel to American Pie and American Pie 2
the third (originally intended final) installment in the American Pie series