Homer comes in with the mail. He uses the George Foreman mail sorter to find out what's good and what's bad. The bad pile is then grilled into a patty, which Homer eats on a bun. In the good pile is an invitation for the family to spend a free weekend at Stagnant Springs Spa.
Marge enjoys a mud bath and neck massage from turtles. Homer ducks in for a steam. A mystery man sneaks by and turns the steam room thermostat from "Invigorating" to "Murder," and bars the door shut with a wrench. Homer begins to sweat profusely, then passes out. Later, Krusty opens the door. A prune-like Homer falls out.
Marge and Homer tell Wiggum about the attempted murder. He says they need someone who can understand the twisted mind of a murderer to solve this case, so Sideshow Bob is freed from prison.
Bob starts on the case by following Homer around during a normal day. Homer smashes his car and yells at a tow truck driver. Bob tells him not to make any more enemies. Later, at Moe's, it doesn't seem like Bob is over his Bart-killing lust. A hand sticks a pistol inside the door and shoots at Homer, hitting the pickled egg jar. We see a tow truck outside, then hear it start - sounding just like the tow truck from before - and drive off.
At home, Bob makes a dummy Homer and ties it to a line, which makes it look like Homer is walking around the yard. They'll sit and wait to see who the killer is. That night, Moe attacks "Homer" with a baseball bat. Then Patty hits him with a cinder block. Then Willie jams a hoe in his neck. Then Reverend Lovejoy repeatedly kicks him in the crotch. Then the real Homer tackles the dummy and begins punching it, saying, "Once I kill you, everyone will think I'm the real Homer."
Bob thinks Homer should lay low to avoid the killer. At that moment, people come to his house and inform him that he's been named the King of Mardi Gras. Marge says that means he has to ride around on a float all day. He'll be a sitting duck. Bob says the only reason he won is because he was set up. Somebody stuffed the ballot box. He ignores them both and hits the parade route.
Bob wonders how Homer's float is moving if he never heard the engine start. Wiggum says it's running so quiet because some mechanic tuned it up before the parade. Bob begins to piece everything together. The wrench. The truck starting outside Moe's. The greasy smudge on the spa invite envelope. Homer's run in with a tow truck driver.
It turns out that the brakes have been cut on Homer's float. The float races downhill, straight for the Museum of Swordfish. Bob tries to save Homer by shooting himself out of a cannon on the pirate float. In midair, he grabs Homer by his feet and lifts him up right before Homer's throne is impaled on a swordfish. They fly away on the Duff hot air balloon. Another gunshot rings out. It hits Duffman. They look for the shooter. It's a man on stilts. Homer and Bob drop from the balloon and steal stilts from Jimbo and Kearney. They begin chasing the shooter. They corner him in an alley. It's Junior, Homer's mechanic. He removes his mask to reveal that he's Frank Grimes, Jr. The police show up in time to arrest Junior Grimey.
Before heading back to prison, Bob tries to kill Bart, but can't. Instead, he breaks into a song about how he's become accustomed to Bart.