Bart buys an extravagant belt on a trip to the mall, but the school bullies beat him up and steal it. When Homer goes to confront the parents of the boys in Moe's Tavern he promptly receives a beating of his own. During the beating, however, Homer remains completely unfazed; Moe sees this and proposes to Homer that he test out his abilities in boxing.
While shopping with his family at a gadget store, Bart overhears Comic Book Guy failing to return his small-sized gimmicky belt because he does not have his proof of purchase. Bart obtains it from him for $4. He shows off its features to his classmates, but is then chased and beaten by the Springfield Elementary bullies, who steal the belt. Homer meets with the bullies' parents at Moe's Tavern, but they in turn attack him. Despite the force used against Homer, he does not fall down (explained by Dr. Hibbert as an ability gained from a genetic defect). Moe is impressed enough to reveal that he used to be a boxer, and agrees to coach Homer. There are two problems - Homer's punches are especially weak and his stamina is so low that one punch completely exhausts him - so Moe decides that Homer should wait for his opponents to get exhausted from pummeling him, then push them over.
Homer wins his early matches, mostly against underfed hobo opponents who don't put up much of a fight.
Eventually Lucius Sweet, Moe's former boxing manager, arrives with a proposition for Moe; the two arrange a match between Homer and the powerful boxing champion Drederick Tatum in a massive star-studded event. Tatum brings so much power to the ring that he knocks out opponents in one blow, which bores spectators, though Homer's amazing resilience means he could offer an actual fight. Despite a promise to Marge to stop the fight if Homer's life is endangered, Moe also promises Sweet that Homer can endure three rounds. Just seconds into the fight, it becomes clear that Tatum is brutal and inexhaustible, meaning that Homer is going to get killed. Homer realizes the original plan is not going to work and decides to punch his opponent, but misses. Just as Tatum is about to strike a possibly fatal blow, Moe airlifts the badly-beaten Homer from the ring using Fan Man's paramotor. Outside, Tatum expresses his respect for Moe's affection towards Homer, and Sweet declares Moe a perpetual loser, but gives him a healthy check nonetheless. During the credits, Moe, still with the paramotor, is shown saving people from natural disasters in different countries.