After their first year in separate colleges, the four friends come together once more to brainstorm their summer plans. Seeking inspiration, Kevin (Thomas Ian Nicholas) turns to his brother for advice, who suggests renting a beach house where they can throw numerous parties, culminating in a grand finale event. They all agree, but soon realize they’ll need an additional person to chip in. Enter Stifler (Seann William Scott), the wild and oversexed college jokester, who is invited to join the gang.
Upon arriving in Grand Harbor, the friends manage to secure work painting a house, akin to the "Tuition Painters" franchise. The homeowners turn out to be two attractive women, and Stifler concocts a theory that they might be lesbians. In a comedic twist, while the women leave, he sneaks inside to gather evidence, but they return unexpectedly and catch the guys hiding. Instead of calling the police, they decide to have a little fun at the guys' expense. In an outrageous turn of events, Jim (Jason Biggs), Stifler, and Finch (Eddie Kaye Thomas) find themselves in embarrassing scenarios, performing “like for like” sexual acts in exchange for the chance to watch the women do the same. Meanwhile, Oz (Chris Klein) and Kevin take turns listening in on the antics from a ladder with a walkie-talkie, leading to a hilariously disastrous situation as the neighborhood buzzes with overheard conversations.
The chaos continues as multiple storylines unfold. Jim discovers that his high school crush, Nadia (Shannon Elizabeth), is visiting in August. With self-doubt about his past performance, he reaches out to Michelle (Alyson Hannigan) for a review of their prior encounter. At band camp, he embarrassingly pretends to be someone he's not, but Michelle offers to help him improve. Misadventures ensue when Jim accidentally glues his hand to his genitals, leading to an emergency hospital visit. As Nadia shows up unexpectedly, eager for romance, Jim devises a plan alongside Michelle to pretend they’re dating to buy himself time to recover. As they fake their relationship, the lines between performance and reality blur, and Jim realizes he has genuine feelings for Michelle. In a climactic moment where he showcases his trombone skills during a recital, he ultimately chooses her over Nadia, arriving at the big party with Michelle as his official girlfriend.
Meanwhile, Oz grapples with his feelings for his girlfriend Heather (Mena Suvari), who is studying abroad. They engage in comical phone sex, though their efforts are thwarted by various interruptions. However, Heather surprises Oz by returning early just in time for the party. Finch has taken up the art of Tantra and awaits a reunion with Stifler’s Mom (Jennifer Coolidge), reflecting on previous escapades. As the party unfolds, there are hints of his disillusionment, and he ends up surrounded by women but without any physical connections, seemingly content nonetheless.
On the periphery, Matt (Eli Marienthal), Stifler's younger brother, faces a dull party night while tasked with peering out for cops. But boredom turns to excitement when he inadvertently connects with the two women thought to be lesbians, creating a sequence of comedic misunderstandings that bring more fun to the gathering. As the night wears on, Kevin deals with his feelings regarding his ex, Vicky (Tara Reid), ultimately finding closure, while Sherman (Chris Owen) unexpectedly charms Nadia leading them to a spontaneous encounter.
As morning dawns after the chaotic party, the group is packing up to head home when a car pulls up, revealing Stifler’s Mom behind the wheel. In a shocking turn of events, Finch quickly jumps in, leaving Stifler unaware of his friend’s escapade as they drive away together, hilariously setting the stage for future antics.
