An excited Milhouse runs to the Simpsons’ house holding a box in his hand. Homer opens the front door, hitting Milhouse in the head and knocking him out cold. Four hours later, with vultures circling overhead, Bart finds Milhouse’s body, and the box at his side. The box contains a Psycho Cycle Conversion Kit, which customizes a child’s bicycle, making it resemble a Hell’s Angels Harley. Bart talks Milhouse into “trying out” the kit on his bike first. Soon after, Bart and Milhouse ride off on Bart’s tricked-out bike. The coolness factor comes to a crashing halt, however, when the boys encounter Nelson and other bullies riding ten-speed racing bikes. The bullies laugh at Bart’s bike, steal some tassels off the handlebars, and ride off. Bart declares that the only thing he wants in life is a ten-speed bike. When Bart returns home, Homer tells him that he’ll get a new bike only when his old one stops working. Looking mischievous, Bart deliberately places his bike in harm’s way by hanging it from a tree and allowing it to swing out into traffic. Dr. Hibbert accidentally collides with the bike, and offers to pay for a new one. Hibbert then drives away, accidentally running over Snowball II in the process. The Simpsons bury their beloved the cat. Later Homer purchases Bart an expensive Italian racing bike. Unfortunately, it’s unassembled. The salesman offers to assemble it for a small fee, but Homer turns him down. Homer builds the bicycle himself, and when he’s through, Bart is thrilled. When he next encounters the bullies, he moons them while standing on the bike’s pedals. Suddenly the bike falls apart, leaving Bart completely humiliated.

While watching a Robot Wars-style show on television, Homer gets an idea: He’ll make up for the bike fiasco by building Bart a robot. His first attempt is a disaster, but when Bart awakens the next morning, a homemade robot resembling a battered mailbox on three wheels is standing nearby. Bart uses the remote control to make the robot do his bidding. What Bart doesn’t know is that Homer is inside the robot, controlling its every move. Bart enters “Chief KnockaHomer” in a televised boxing match called “Robot Rumble.” Since Homer has to be inside the machine, he finds an excuse not to accompany Bart to the event. Despite being bashed around by its robotic opponent, Buzzkill, Homer manages to pilot Chief KnockaHomer to victory. Later Chief KnockaHomer also defeats MurderPuss, operated by Lovejoy and his daughter; and Ralph-O-Cop, operated by Wiggum and Ralph. Meanwhile Lisa and Marge travel to an animal shelter to pick out a new cat. Lisa chooses an orange cat and names it Snowball III. When she brings it home, however, the animal ends up face-down in the family’s aquarium. Later the family buries Snowball III. Its replacement, a cute white kitten named Coltrane, jumps out a window when it hears Lisa’s saxophone.

Homer grows uneasy when he realizes his next robotic opponent is Killhammad Aieee, a fearsome device built by Professor Frink. As the fight gets underway Bart opens his robot… and sees Homer’s butt crack. Bart tells Homer it’s really cool of him to risk his life against the other robots. Suddenly Round Two begins, and Chief KnockaHomer is dragged back into the ring. As KnockaHomer is destroyed, Homer’s head pops out of the twisted metal. Since Killhammad Aieee is programmed to obey Isaac Asimov’s laws of robotics, however, it cannot harm him. After getting and then losing a succession of cats, Lisa comes to the conclusion that she probably shouldn’t own a new cat. Then a demented Cat Lady tosses a cat at Lisa and moves on. Lisa tells the cat it’s not a good idea to hang around her, because all the other cats ended up dead. The cat makes its way into the street, where a car fast approaches. The car swerves and crashes into a tree. Lisa pats the kitty and says that it has good luck. She names it Snowball II, and says that she’s going to pretend that nothing ever happened to her other cat.