The Simpsons visit a high-tech gadget store and Bart buys a gimmicky utility belt from Comic Book Guy for $4. He shows off its features to his classmates until he is beaten up by Dolph, Jimbo and Kearney, who steal the belt. In response, Homer decides to talk with the bullies' fathers at Moe's Tavern, but he is also beaten; however, they are unnerved when they cannot even knock Homer down, and are then chased away by a shotgun-wielding Moe.
Moe is impressed by Homer's ability to absorb severe punishment and suggests he take up boxing, with Moe – a former boxer himself – as his trainer and manager. Marge is annoyed by the idea, but a medical test diagnoses Homer with a rare genetic abnormality effectively rendering him near-invulnerable to knockout punches. However, while training, it is discovered that Homer is a weak puncher, and is extremely out of shape. Moe strategizes that Homer should stand still in the ring and let his opponents exhaust themselves trying to knock him out, and then knock them down with a tap. Marge insists that Homer get a physical by a competent doctor, hoping he wont pass the requirements for a boxing license. However, Homer easily passes the state requirement to box, wrestle or "be shot out of a cannon", as the state Springfield is in had apparently been founded by circus freaks.
Homer does well in his first fights, mainly because he's competing in Springfields "hobo fight" circuit, against opponents almost as out of shape as he is. At every match, he follows Moe's advice and starts to gain widespread attention, eventually becoming the top ranked fighter in the Springfield Hobo Boxing Association. Moe's former boxing manager, Lucius Sweet visits Moe and announces that the current Heavyweight Champion, Drederick Tatum is being released from prison and is ready for a comeback fight, and Sweet wants Tatum to fight Homer. Tatum has become infamous for scoring one-punch knockouts in his opening move, resulting in matches to which last only a few seconds. Lucius hopes that due to Homer's fame as a human punching bag, he will at least be able to endure a few rounds before being bludgeoned unconscious. Moe, despite knowing full well that Tatum is far too strong for a fighter of Homer's ability, still wants to take his last shot at the big time and promises Sweet that Homer will last three rounds with Tatum.
Tatum is paroled and the media begins hyping the fight. The odds against Homer are 1000–1 and everybody believes that Homer will lose. Even Homer himself visualizes that the only way he could win is if a heart defect fells Tatum before entering the ring. Marge makes Moe promise her that the moment Homer is in any danger, he will throw in the towel. But after Marge leaves, Moe throws away the towel in a garbage can.
The fight starts and it is soon obvious that Homer cannot withstand Tatum's fearsome barrage and is in danger of being knocked out within six seconds. Homer decides to fight back, but misses completely. Moe, seeing Homer get destroyed, flees. As Tatum readies himself to deliver the final blow, Moe flies in using the Fan Man's paramotor and carries Homer away, saving him. Outside the arena, Homer thanks Moe for saving him and Tatum shows respect for the love Moe showed for Homer. Sweet declares that Moe will always be a loser, but still gives him a check for a hundred thousand dollars. Moe starts up the Fanman's paramotor and flies off into the night. During the credits he is seen helping people around the world with the paramotor.