Daniel Eugene "Rudy" Ruettiger grows up in Joliet, Illinois; dreaming of playing college football at the University of Notre Dame. While achieving some success with his local high school team (Joliet Catholic), he lacks the grades and money to attend Notre Dame, not to mention talent and physical size. Ruettiger was much smaller than an average football player, standing just 5'6" (1.68 m) and weighing 165 lb (75 kg).
Ruettiger takes a job at a local steel mill like his father Daniel, Sr. (a Notre Dame fan); he prepares to settle down. But when his best friend Pete is killed in an explosion there, Rudy decides to follow his dream of attending Notre Dame and playing for the Fighting Irish.
He leaves for the campus, but fails to get admitted to Notre Dame. With the help of a local priest (who mistakenly thinks at first Rudy wants to become a priest), Rudy starts at a small junior college nearby named Holy Cross, hoping to qualify for a transfer. He also manages to get a part-time job on Notre Dame's groundskeeping staff and befriends D-Bob, a graduate student at Notre Dame and a teaching assistant at his junior college. The socially-awkward D-Bob offers to tutor Rudy if he helps him meet girls. Suspecting an underlying cause to Ruettiger's previous academic problems, D-Bob has Rudy tested, and Rudy learns that he has dyslexia. Rudy learns how to overcome his disability and becomes a better student. At Christmas vacation, Rudy returns home to his family's appreciation of his report card, but is still mocked for his attempts at playing football and also dumped by his girlfriend, who starts seeing one of his brothers.
During his final semester of transfer eligibility, Rudy is admitted to Notre Dame. He rushes home to tell his family. At the steel mill, his father announces it over the loudspeaker, "Hey, you guys, my son's going to Notre Dame!" After "walking on" as a non-scholarship player for the football team, Ruettiger convinces coach Ara Parseghian to give him a spot on the practice (or "scout") squad. An assistant coach warns the players that 35 scholarship players won't make the roster. But, Ruettiger exhibits more drive than some of his scholarship teammates.
Parseghian agrees to the young man's request to suit up for one home game in his senior year so his family and friends can see him as a member of the team. However, Parseghian steps down as coach following the 1974 season. Dan Devine succeeds him in 1975 and decides against giving Ruettiger a chance to appear at a home game. Led by team captain and All-American Roland Steele, the other seniors rise to his defense and lay their jerseys on Devine's desk, each requesting that Rudy be allowed to dress in their place. Devine relents and lets Ruettiger suit up for the final home game, against Georgia Tech.
At the final home game, Steele invites Ruettiger to lead the team out of the tunnel onto the playing field. As the game comes to an end, and Notre Dame is ahead, Devine sends all the seniors to the field, but refuses to let Rudy play, despite the pleas from Steele and the assistant coaches. Then, the Notre Dame bench starts a "Rudy!" chant that soon goes stadium wide, and the offensive team, led by tailback Jamie O'Hare, overrules Devine's call for victory formation and they score another touchdown instead. Devine finally lets Rudy enter the field with the defensive team on the final kickoff. He stays in for the final play of the game and sacks the opposing quarterback, and is carried off on the shoulders of his teammates.