The film begins with Jim Levenstein (Jason Biggs) and Michelle Flaherty (Alyson Hannigan) having dinner, celebrating their college graduation. They've been dating for 3 years now. Jim intends to propose to Michelle, but receives a phone call from his dad saying that Jim forgot the engagement ring at home and his dad is on his way to bring it to him. As Jim tries to stall, he talks about the next step which Michelle interprets as spicing up their sex lives. She gets the impression that he wants a public sex act and proceeds to perform oral sex on him under the table. When Mr. Levenstein (Eugene Levy) arrives, Jim tries to hide the fact that he's being fellated. Not knowing Michelle is under the table, Jim's dad expresses his excitement at the proposal. Then, after Michelle bumps her head under the table out of shock , all chaos ensues as Jim ends up exposed and has to hike his pants up quickly. Saving face, he swallows what's left of his pride and asks Michelle to marry him which she readily agrees.Jim has a night at the local bar with Michelle, Kevin(Thomas Ian Nicholas) and Finch (Eddie Kaye Thomas) to celebrate their engagement. During that night, it is revealed that Michelle likes to dance but Jim is a terrible dancer. The first thing about the wedding that everyone agrees to, is that Stifler (Seann William Scott) is not invited and should be kept away from the wedding. This proves to be impossible when Stifler drives by Jim's house and decides to crash what he thinks is a graduation party but is actually the engagement party. He messes with the cake which ends up on his lap and Jim's as well. Jim tries to take his pants off to clean them and chaos ensues ending with Jim looking like he's having sex with dogs when Michelle's parents (Fred Willard and Deborah Rush) meet him for the first time.When Stifler realizes that Jim is getting married, he volunteers to organize the bachelor party which Kevin and Finch hadn't thought about but agrees that Stifler would be the perfect guy for such an occasion. They don't tell him however that he is not invited.Afterward, Jim worries that the wedding will be a disaster. Michelle's parents hate him after the dog incident. Michelle is stressed but doesn't want to tell Jim about her worries so Jim doesn't know what she really wants and he knows she'll want to dance and he's really bad at it. He shares these details to Kevin and Finch who offers their help. Looking for dancing lessons, Kevin and Finch meets Stifler and try to hint that he's not invited at the wedding. Jim goes to Stifler to try to explain why he's not invited. Stifler gets angry and screams at Jim while his football team surrounds them and scream with him. Stifler uses that moment to whisper that he knows how to dance and he can teach Jim. They have a first lesson together and Jim finally agrees to invite Stifler and allow him to take care of the bachelor party.Meanwhile, Finch has been following Michelle under disguise to learn more about her desires for the wedding. He finds out that Michelle's dream wedding dress is made by only one designer working for one store in Chicago. So Jim sets out to find the dress maker with the guys and Stifler. They go to Chicago but the dress maker named Leslie isn't at the store. One of her colleagues agrees to give them the address of a few bars where they could find her. They split to cover more territory and Stifler ends up in a gay bar where he gets in an argument with a big man nicknamed Bear who happens to know Leslie. Bear sends them away refusing to help them. As Jim is losing faith in ever finding Leslie, Stifler returns and has a dance-off to make up for his rudeness towards Bear. Leslie, the man who was next to Bear all along finally reveals himself and agrees to make the dress.Michelle's sister Cadence (January Jones) arrives to help for the preparations, and Finch is quickly attracted to her. Sadly, so is Stifler. Upon hearing that Cadence is hoping to attract a decent guy, Stifler adopts a more meek attitude and acts like Finch, and avoids swearing and speaks of philosophy. The act works as it seduces both Cadence and her mother. When Finch realizes that Cadence is beginning to tire of the intellectual Stifler however, he starts to act immature, rude, and perverted: in other words, like the real Stifler. This change of attitude momentarily gets him the attention and favor of Cadence. The dress episode has convinced Michelle's parents that they should give Jim a second chance and they schedule a private dinner. Jim lies to Stifler that everyone will be gone that night so he can have his private moment with Michelle's parent. Unfortunately, Stifler, thinking that Jim will be home alone, sees it as the perfect moment for the bachelor party to take place. Stifler arrives at Jim's house with Kevin, Finch, Bear and his "girls": Fraulein Brandi and Officer Krystal. Not waiting for Jim, they get the party started and play Bondage and Submissive roles with the girls. When Jim arrives with Michelle's parent, Stifler and Finch realizes their mistake and they hide everyone. They almost manage to get away with it until they find Kevin almost naked and tied to a chair in the closet. Stifler makes up an excuse to explain everything and tries to take the blame for it but Jim insist to take the blame himself which convinces Michelle's parents that Jim is willing to make a lot of efforts for their daughter and they give him their blessing.Because he is now her favorite best man, Michelle's mother entrust Stifler to protect the bride's wedding ring until the ceremony. Not long after arriving at the hotel where the wedding will take place in a few days, Stifler looses the ring when he unintentionally feeds it to the dogs. He spends the whole day with them hoping to catch the ring when they defecate. When he finally gets the ring back, he tries to sneak to his room to wash it but is intercepted by Michelle's parents. Mistaking the stool in Stifler's hand for a truffle, Michelle's mother, who loves chocolate, attempts to eat it, giving Stifler no other choice but to eat it himself to save face. Only Finch knows the truth and reacts accordingly.Later, Cadence begins to catch on that Stifler may not be all he says he is, but finds herself still attracted to him so she proposes him a sexual encounter. Stifler picks up a bottle of champagne on his way to his encounter with Cadence but accidentally turns off the refrigerator holding the flowers for the wedding. He gets in an argument with Finch and goes into full Stifler mode just as Cadence walks by and sees everything. She obviously cancels the encounter.Another crisis erupt when Jim's grandmother express her disapproval of the wedding and hates Michelle because she isn't Jewish, a fact that deeply saddens Michelle. Jim tries to explain to her that his grandmother is very old fashioned but Michelle still takes it very personal.Because the refrigerator was off, the flowers have all overheated and died. With everyone angry at him, Stifler leaves the hotel angry and sad. On his way home he sees the flower shop that provided the flowers for the wedding. He finds the owner's address and wakes her up in the middle of the night. She claims the job is impossible since she had four people working for two days to do the first job. By using an old speech from his coach in the first movie ("you don't score until you score...") he convinces her to help him redo the flowers. To do so, he also brings in the football team he coaches to set up the entire room with new flowers. Jim and Michelle, who were very sad the night before, wakes up to the sound of Stifler leading his whole team into decorating the wedding hall. Cadence is impressed and agrees to forgive Stifler and be his date for the wedding.Finch admits that Stifler was probably better for her anyway. Stifler, angry that he can't hate Finch after such a noble remark, tells Finch that he hates not hating him. Finch reminds him that he slept with Stifler's mom twice. Stifler says "Thank you" as a way to reveal that all the hatred is back.Just before the ceremony, Stifler and Cadence plan to have sex in a linens closet. Cadence is intercepted by her mother and is unable to sneak away while Jim's grandmother gets dumped in the linens closet by the ushers because they found her annoying. Stifler enters the dark closet and thinks he's with Cadence and have sex with her until Finch and Kevin finds them and Stfler realizes he was doing Jim's grandmother who insist he finishes the job. Michelle and Jim finally get married. Everything goes smoothly, even Jim's grandmother is all smile now, thanks to Stifler. At the reception, Jim impresses everyone with his new dancing skills taught to him by Stifler. They invite everyone to join them on the dance floor. Everyone is dancing and having a good time except Finch who's sitting all by himself until Stifler's mom (Jennifer Coolidge) shows up. They tell each other they are over each other but she invites him to her room. The movie ends with Stifler's mom and Finch in the tub having sex as the milf guys (John Cho and Justin Isfield), are watching in awe from the window.