When Homer receives an invitation to spend a free weekend at Stagnant Springs Spa, the family heads for the desert for facials, yoga and relaxation. Homer decides to try out a steam room, only to find himself trapped inside. Krusty finds a wrench lodged in the steam room door handle, and when he removes the wrench, a withered Homer falls out of the steam room. Later, the Simpsons discover that the invitation they received in the mail is a forgery. Homer realizes that someone is trying to kill him. He turns to Chief Wiggum for help, but Wiggum thinks only someone with the twisted mind of a murderer could hope to solve the case: Sideshow Bob.

Wiggum tells Sideshow Bob that if he helps solve the mystery, the government will make it worth his while. Bob asks for round-the-clock access to the family…and especially Bart. Bart reacts with understandable concern, as Bob has tried to kill him numerous times. But Wiggum has Bob outfitted with a special “shock garter,” which allows the family to shock him using a remote control device. Once the device is attached, and Bob is allowed access to the Simpson home, he receives permission to follow Homer around during the course of a normal day to determine who might be out to cause him harm. Homer and Bob travel to the Kwik-E- Mart, where Apu welcomes them to Springfield Mardi Gras headquarters.

Later, the pair drive to a gas station, where Homer has his car repaired by a man named Junior. And finally, they stop by Moe’s bar, where someone takes a potshot at Homer. When Homer returns home, Bob sets a trap for the killer by hanging a dummy dressed as Homer from a clothesline. That night, a slew of people who dislike Homer attack the dummy, but none of them turns out to be the mysterious killer. When Bob examines the spa invitation more closely, he notices a black smudge, but has no idea what it means. Later, Bob tells Homer that his best chance of survival is to stay out of sight. A short time later, revelers tell Homer he’s been elected king of Mardi Gras. Marge warns that Homer will be required to ride on a float all day…making him a sitting duck.

Bob determines that Homer was elected king because someone stuffed the ballot box with identical cards with Homer’s name written on them. Bob warns that Homer has been set up, but Homer decides to ride the float nonetheless. Later, Bob learns from Wiggum that a mechanic tuned up Homer’s float short before the parade got underway. It suddenly dawns on Bob that all of the pieces of the puzzle point to one person: Junior, the man who fixed Homer’s car at the garage. Bob warns Homer that the float’s brake line has been cut. Dangling from the Duff blimp, Bob pulls Homer off the float before it crashes into a swordfish museum. A short time later, Bob and Homer corner the mechanic in an alley. The mechanic identifies himself as Frank Grimes, Jr., who is the son of the man who hated Homer and ended up dying. The mechanic is taken into custody. Later, a knife-wielding Bob corners Bart in his bedroom. But Bob has a change of heart and realizes he’s grown accustomed to Bart’s face…and can’t bring himself to kill the boy. Bob hops out a window and escapes into the night.