Bart is taunted by school bullies Nelson, Jimbo, Dolph and Kearney, because he does not have a 10-speed bike like they do. In order to get Homer to buy him a 10-speed bike, Bart gets the bike he already has run over by Dr. Hibbert's Mercedes-Benz G500. Homer buys the 10-speed bike for Bart, but refuses to pay the small assembly fee and builds it himself. Bart is happy since it looks great and works perfectly at first, but it falls apart when he moons the bullies. Homer, wanting Bart to be proud of him, tries to build a battle robot for the "Robot Rumble". He fails to construct one, but then decides to build a robot with himself in it; Bart names the robot "Chief Knock-a Homer". Unaware of Homer's ruse, Bart enters the robot in the Rumble.
Meanwhile, Dr. Hibbert's car runs over and kills the Simpsons cat Snowball II shortly after crushing Bart's bike. A devastated Lisa recites a poem tearfully at the funeral, where Snowball II is buried next to Snowball I. Lisa gets a ginger cat, which she names Snowball III, but he drowns in the fish tank. The next cat, Coltrane (Snowball IV), jumps out of a window after hearing Lisa play her saxophone. Because of this the owner of the cat sanctuary refuses to give Lisa any more cats. As Lisa sits outside brooding over the recently deceased cats, the Crazy Cat Lady shows up, screams nonsense and hurls a cat (which has a striking resemblance to Snowball II) at her. At first Lisa tries to drive it away, fearing it will meet the same fate as her other cats, but this cat proves to be more durable, when an oncoming car driven by Gil Gunderson swerves and crashes into a tree (this proves good for Gil because he has insurance). Lisa elects to keep the cat and says "You're Snowball V, but to save money on a new bowl, let's just call you Snowball II and pretend this whole thing never happened." In an instance of meta-humor, Principal Skinner then stops by and asks Lisa "That's really a cheat, isn't it?" Lisa replies "I guess you're right, Principal Tamzarian." Skinner walks away defeatedly, acknowledging the new cat as "Snowball II."
Homer defeats numerous opponents to make it to the finals, but he is physically injured from the battles with the other robots, and at one point he finds a large piece of jagged metal in his arm. In the final match against Professor Frink's undefeated super-robot, which looks like the ED-209 enforcement droid from the RoboCop films, Bart finds Homer in the bot after the gruelling first round. Caught, Homer apologizes to Bart, but Bart is impressed because of all the pain Homer went through to win his son's admiration. In the second round, ED-209 squeezes Homer out of the robot, but immediately stops as soon as it sees him. Frink explains that the robot follows Isaac Asimov's Three Laws of Robotics and "is programmed never to harm humans, only to serve them." ED-209 sets out a comfy seat for Homer and pours him a martini. Homer wins and Bart is proud of his dad.