At Faber College in 1962, freshmen Larry Kroger and Kent Dorfman attend pledge week, hoping to join a fraternity. Wanting to pledge Omega House, the most prestigious fraternity on campus, the boys attend the invitational party but quickly realize that the Omegas won’t accept them. Although Kent suggests Delta House instead, because his brother is a former fraternity member or “legacy”, Larry laments that Delta has a terrible reputation. At Delta House, the wild and crazy members are more welcoming to Larry and Kent. There, they meet John “Bluto” Blutarsky, who is the sergeant at arms, Robert Hoover, the chapter president, Eric “Otter” Stratton, the rush chairman, as well as Donald “Boon” Shoenstein and his frustrated girlfriend, Katy. The next day, Greg Marmalard, president of Omega House, meets with the school’s dean, Vernon Wormer, who wants Delta House expelled from Faber College for breaking campus rules and retaining a low collective grade point average. Dean Wormer orders Marmalard and his fellow Omega, Doug Neidermeyer, to expedite the expulsion. Meanwhile, Delta House review fraternity brother pledge candidates and accept Larry and Kent, mainly because they are in need of annual dues. At the Delta initiation, Bluto dubs Larry “Pinto,” and Kent “Flounder.” The fraternity celebrates with a wild party. Meanwhile, at Omega House new pledges are spanked in the dark while senior members look on. Sometime later, Boon, Katy and Pinto go to the home of English professor, Dave Jennings, where they all smoke a marijuana cigarette. During Reserve Officers’ Training Corps (ROTC) practice, Otter and Boon see Neidermeyer bully Flounder and decide to exact revenge. As Neidermeyer forces Flounder to clean the stalls of Neidermeyer's beloved horse, he continues to abuse the Delta member. Later, Bluto and his fellow Delta brother, Daniel “D-Day” Simpson Day, convince Flounder to sneak Neidermeyer’s horse into Dean Wormer’s office late at night with a gun. Unbeknown to Flounder, the gun is filled with blanks so he fires at the ceiling. The prank backfires when the horse dies of a heart attack at the sound of the explosion. The next day at the cafeteria, Otter flirts with Marmalard’s girlfriend, Mandy Pepperidge, in front of Marmalard and other Omegas. Continuing his pranks, Bluto spits food in Marmalard’s face and a food fight ensues. Later, Bluto and D-Day steal the answers to an upcoming Psychology mid-term exam, unaware that members of the Omega House have planted a fake test with the wrong answers. When all of the Deltas fail the exam, Dean Wormer informs them that he will revoke the fraternity’s charter if they make one more mistake. Hoover wants to prevent the expulsion, but Otter and Boon think the fraternity is doomed and decide to throw one last toga party. The Deltas go to the grocery store for supplies. There, Boon and Pinto shoplift while Otter flirts with Dean Wormer’s wife, Marion. Pinto invites the young cashier, Clorette DePasto, to the party. Back at Delta House, the band Otis Day and the Knights play for raucous partygoers. Marion arrives, intoxicated, and has sex with Otter, but Clorette passes out before Pinto can seduce her. He returns an unconscious Clorette to her parents’ house, where her father, Mayor Carmine DePasto, finds his daughter in a shopping cart. Meanwhile, Marion, still intoxicated, returns home to Dean Wormer. Furious, Dean Wormer organizes a campus tribunal to try Delta House, but he does not let the fraternity members defend themselves. The Dean revokes Delta’s charter and threatens to expel the members. Sometime later, the Deltas go on a road trip to Emily Dickinson College to pick up girls. Upon meeting Shelly Dubinsky, who tells Otter that her roommate, Fawn Liebowitz, recently died, Otter pretends to be the deceased girl’s fiancé. Otter convinces Shelly and her sorority sisters to go out with the Deltas to “cheer him up.” At the Dexter Lake Club, they watch Otis Day and the Knights perform and find themselves the only white people in attendance. When some of the African-American men threaten the boys, they run out of the club, leaving the girls behind. Back at Faber College, Boon tries to make up with Katy, but he discovers that she is having an affair with Professor Jennings. Later, Mandy’s friend, Babs Jansen, sets up Otter for an ambush by the Omegas, telling him that Mandy wants to have a sexual rendezvous. When Otter arrives at a motel to meet her, the Omegas attack him. Dean Wormer expels the entire Delta House for failing their mid-term exams and notifies the local draft board. Although most of the Deltas feel defeated, Bluto encourages them with an impassioned speech and they decide to get revenge by invading the annual homecoming parade. Manning an undercover float, the Deltas create chaos and destruction at the event and wreak havoc on Dean Wormer and the Omegas. In the future, Delta members rise to success while Omegas meet tragic ends.