In the autumn of 1962, freshmen Larry Kroger and Kent Dorfman are determined to pledge a fraternity at Faber College. After facing rejection at the elite Omega Theta Pi house party, they decide to explore the nearby Delta Tau Chi house, where Kent's status as a "legacy" due to his brother Fred's membership grants them entrance. During this vibrant party, they encounter a motley crew that includes the exuberant John Blutarsky (nicknamed "Bluto"), the charming chapter president Robert Hoover (known as "Hoov"), smooth-talker Eric Stratton (often referred to as "Otter"), the free-spirited motorcyclist Daniel Simpson Day (affectionately called "D-Day"), Duck Donald Schoenstein (known to friends as "Boon"), and Boon's exasperated girlfriend, Katy. Larry and Kent are welcomed into the Delta family, earning the fraternity names "Pinto" and "Flounder."
Simultaneously, Dean Vernon Wormer has his sights set on disbanding the Delta fraternity due to their poor academic performance and multiple conduct violations. He enlists Omega's president, Greg Marmalard, and ROTC Cadet Commander Douglas C. Neidermeyer to scheme against the Deltas. Tensions escalate when a retaliatory prank leads to the accidental death of Neidermeyer's horse. Unbeknownst to the Deltas, a classic fraternity prank involves Bluto and D-Day taking the answer key to an impending exam, only to fall victim to a switch orchestrated by the Omegas, leading to catastrophic grades for the Deltas.
Undeterred by their academic failure, the Deltas throw a legendary toga party, luring Pinto to swipe supplies from a local supermarket. Here, Pinto encounters Clorette, a young cashier, and invites her to join the festivities. Meanwhile, Otter’s flirtation with an unsuspected older woman reveals her to be Dean Wormer's inebriated spouse, Marion. As the toga party unfolds with Otis Day and the Knights performing, things take a twisted turn. Pinto’s night with Clorette ends in a moral choice when he decides against an act of violence after she passes out drunk, later discovering her shocking connection to the town mayor.
Amidst the chaos, Wormer initiates a kangaroo court, stripping the Deltas of their charter and fencing off their possessions. In a bid for freedom, Otter, Boon, Pinto, and Flounder embark on a road trip in a borrowed Lincoln, arriving at an all-girls college where Otter fabricates a story to charm dates. Their escapades at a local bar expose them to the fallout of their presence, leading to their frantic retreat.
As tensions rise within the fraternity, Boon finds Katy in a compromising position, igniting further discord. Meanwhile, Wormer, reveling in the Deltas' unraveling, expels them and notifies the military of their eligibility for service. The Deltas, fueled by rebellion and camaraderie, plan their ultimate revenge during the Homecoming parade, dramatically transforming Fred's Lincoln into the "Deathmobile." The parade descends into chaos, culminating with the Deathmobile crashing into the reviewing stand, launching Wormer, Marmalard, and others into disarray.
In the aftermath of this riotous spectacle, the futures of various characters unfold, revealing a mix of triumphs and tragedies, with Bluto surprisingly becoming a U.S. senator, while the Omegas bear the unfortunate consequences of their scheming. The tumultuous journey through college life encapsulates the essence of brotherhood, rebellion, and the inevitable shifts of fate.
