While on a date in a fancy restaurant with Michelle, Jim is ready to propose to her. His father calls to inform him that he has the ring. Michelle misinterprets when Jim stalls the question. Jim's dad arrives as Jim is receiving fellatio, and a mishap grasps the restaurant's attention. Jim proposes and Michelle accepts.
Jim wants to exclude Stifler from the wedding, having concerns that Stifler's behavior isn't socially acceptable. When Jim is worried about learning the dance, Stifler agrees to teach Jim to dance, and over time to tone down his obnoxious personality, in exchange that he be allowed to attend the wedding and host a bachelor party. The boys go on a road trip when they discover there is only one designer who makes the dress that Michelle wants. They go to Chicago looking for "Leslie Sommers"; Stifler unwittingly walks into a gay bar, and his raucous behavior gets him into a dispute with several patrons. Stifler challenges Bear to a dance-off, wins, and Bear offers to provide strippers for the bachelor party. Leslie reveals "him"self and agrees to make the dress for Michelle. Michelle's parents, who were initially skeptical of Jim, agree to have a preliminary 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 and mannerisms. Stifler arranges the bachelor party, including strippers, for everyone at Jim's house except Jim, who unknowingly has arranged dinner with Michelle's parents there. The party is abruptly halted by the unexpected return of the three. 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, stripped to his boxers and tied to a chair. The boys explain that it was an attempt to make Jim seem like a hero that went wrong, and Michelle's parents accept this and tell him that if he puts that much effort into the upcoming marriage, she can give her blessing.
Michelle has doubts regarding that Jim's paternal grandmother dislikes Michelle for not being Jewish, and about Stifler's invite to the wedding. 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 lay down Cadence. Previously, Stifler, unaware of Cadence's presence, had revealed his true rude and obnoxious personality. When Stifler later returns, the flowers are dead. Enraged, 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, much to the amazement of Jim and Michelle. 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 to back him up.
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 placed Jim's grandmother in that closet to sustain her hostility about the wedding; Stifler only realizes this upon walk-in by Finch and the ushers. 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 is sitting by himself when Stifler's mom arrives. Although agreeing they are over each other, Stifler's mom mentions having a double suite and invites Finch to join her. The film ends with John and Justin spying on Stifler's mom and Finch in her suite's couple-size bathtub, having oral sex.
What does Quickly hear while returning to the hotel?
Someone in the supply closet and steps inside