Larry Kroger and Kent Dorfman are freshmen roommates at Faber college, going through fraternity rush. Rejected by the prestigious Omegas (who call them "a wimp and a blimp"), they wind up at the notorius Delta house. ("Great," says Larry, "I heard Delta's the worst house on campus!") There, they meet the Deltas, including house president Hoover (who tries but fails to be straight-laced), womanizer Otter, "the animal" Bluto (played by John Belushi), Harley posterboy D-Day, and Boone (in love with nice-girl Katie). As the Omega pledges suffer paddlings, our twosome (newly christened "Pinto" and "Flounder", respectively) drink beer and sing "Louie, Louie".
Enter the college's Dean Wormer, who conspires with Omega president Greg to stamp out the Deltas. ("No more fun of any kind!") He places the Deltas on "double-secret probation", basically a spy campaign by the Omegas. For example, when the Deltas try to steal answers to an upcoming exam, the Omegas get there first, switch exams, and the Deltas wind up with every answer wrong. Realizing they are doomed, the Deltas decide to go out with a bang, and throw a Toga party. Katie unsuccessfully lobbies Boone to skip it, on the grounds that he is "too well to attend". At the party, Pinto seduces the mayer's daughter, Otter ravages the Dean's wife, and Bluto pours mustard on himself, all to the music of Otis Day and the Knights.
The Dean moves swiftly, setting up a kangaroo court which "decides" to seize Delta's house and revoke their charter, forcing the frat brothers to go underground. Stealing Flounder's brother's car (over Flounder's vocal dissent), Otter, Boone, Pinto and Flounder make a road trip to Dickenson Girl's College. Otter gets them "sympathy dates" by posing as the fiancee of Fawn Liebowitz, a sophomore killed the previous week in a kiln explosion. They go to a club where Otis Day and the Knights are playing, only to discover they are the only white people there. Otter gets his date alone in the car and sweet talks her clothes off. The others lose their dates to guys in the club and beat a hasty retreat, catching Otter "en flagrante delecto".
Back at school, the Dean gets the Deltas' midterm grades, and summarily expels them all. ("GPA 1.2-- congratulations, Kroger... you're at the top of the Delta pledge class.") Upon recieving the news, Bluto remarks, "Great; 12 years of college down the drain." He soon gives a rousing speech, inspiring the others to get even. ("Was it over when the Germans bombed Pearl Harbor? Hell no!") And the upcoming Homecoming gives them their perfect opportunity...