1962. Larry and Kent are new students at Faber College, where they hope to join a fraternity. After striking out at Omega, the duo visit Delta, a chaotic party frat where the beer is always flowing. Kent is a legacy at Delta, so believes he has a better chance of getting in. The Delta brothers include the handsome ladies' man Otter, committed relationships guy Boon, motorcycle owner D-Day, chapter president Robert Hoover, and the wild card party guy, Bluto. Larry and Kent pledge and take on new fraternity names, "Pinto" and "Flounder."
Meanwhile, Dean Wormer is intent on getting the Delta brothers kicked out of school, so fed up is he with their debaucherous ways. Consulting with Greg Marmalard, the preppy president of Omega, Wormer puts Delta on "double secret probation." The Deltas continue to cause mischief, accidentally killing the ROTC cadet commander's horse. Boon and Katy, his girlfriend, face some problems, as she wants him to settle down and become more serious, but he wants to party it up in his final days at college.
Bluto and D-Day steal the answers for a Psychology midterm, but the Delta brothers all fail, because the answers were actually wrong answers planted by the members of Omega. As a result, the Delta's grades drop significantly, and Wormer threatens to expel them if they don't clean up their act. Nevertheless, the Deltas throw a wild toga party. Wormer's wife, whom Otter meets at the supermarket, attends the party and sleeps with Otter. Meanwhile, Pinto makes out with the mayor's daughter.
Wormer continues to level punishments against the Deltas, taking items from their house after the toga party. Otter, Flounder, Pinto, and Boon all go to the nearby Emily Dickinson College to find dates, and attend a bar where Otis Day, a soul singer, is playing. The group of college students are the only white people in the bar, and eventually the boys' dates get stolen by a group of patrons.
After finding out that Otter has been sleeping with his girlfriend, Mandy Pepperidge, Greg Marmalard lures him to a hotel and beats him up. After Wormer expels the Delta brothers, they become intent on exacting revenge. Outfitting an old beat up car into a parade float, they create chaos at the homecoming parade.
