Homer and Marge are in bed about to have sexual intercourse, but Homer is too overweight to do so. When he arrives home for dinner the next evening, Homer is surprised to see that Marge has hired a formerly obese nutritionist named Betsy Bidwell. She puts Homer on a diet for which he should solely eat bell peppers. Although he appears to eat nothing but bell peppers for quite a while, he gains seven pounds. Marge suspects Homer of cheating on his diet. Later, while the kids watch TV, Marge sees an advertisement for a television show named Sneakers, a parody of Cheaters designed for couples who are cheating on each other. Someone on the advertisement says to call the Sneakers hotline if they suspect cheating, which Marge realizes she can use to find out about Homer.
Bart and Lisa, sent out of the house by Marge so she can make an "adult phone call" to Sneakers, tour Springfield National Park. At the top of a mountain, the two discover Martin excavating for arrowheads. While Lisa joins him in excavating, a frustrated Bart plays a prank on Martin. Stealing Sideshow Mel's bone (revealing that he actually has long flowing hair), Bart buries it. Martin arrives thinking it is a real artifact. Bart tugs on a string connected to the bone, which flies up to knock Martin in the head. He stumbles over the edge of the cliff, and falls onto a smaller ledge. Lisa takes a long stick and tells him to grab onto it, but inadvertently knocks him off the ledge, and he falls into the trees at the bottom of the cliff. The two return home believing that they have killed Martin, and feel guilty and unsure what to do. On the news, Kent Brockman reports Martin has disappeared and is presumed dead. Chief Wiggum says when Martin landed he was eaten by a cougar. Bart, feeling guilty, wants to admit his crime to pay the price, but Lisa, frightened of going to jail for being an accomplice, convinces him to keep quiet. Homer is about to leave the house, claiming he is going to work on a Saturday. Marge calls Sneakers which prompts a nearby van full of Sneakers agents to stalk Homer.
A memorial is held as a tribute to Martin in the school gym. Nelson realizes that Martin had a fear of heights so it would be unusual for him to be at the top of a cliff. Nelson travels to Springfield National Park and discovers Sideshow Mel's bone on a string. Meanwhile, the Sneakers agents catch a video of Homer going to a restaurant. Marge confirms her suspicions of Homer but realizes that the reality show people do not care about their marriage (and if anything, are trying to destroy it). She quits the show and embraces Homer, while the host goes off and edits the episode to make Marge look crazy.
Nelson suspects Bart and Lisa's involvement in Martin's disappearance. Having read Martin's diary, Bart heads to Martin's greenhouse to complete his butterfly project. Lisa follows him and they finish the project. The guilt becomes too unbearable and Lisa confesses to the murder out loud. Nelson appears and catches Lisa's confession on tape, just as Martin emerges. He recounts how he survived the fall thanks to his underwear with an extra-durable wedgie-proof waistband. His waistband caught on a tree branch and then the cougar pulled at his clothes, tossing him to a safe place. Lisa concludes that underneath her innocent appearance is a dark, twisted person. The ending imitates an opening to the NBC Mystery Movie, with Nelson Muntz as Columbo, Dr. Hibbert as Quincy, M.E., Rich Texan as McCloud, and Mr. Burns and Smithers as McMillan & Wife.