Bart plays a prank on Martin, but by accident, Martin falls down a cliff. Bart and Lisa start to feel guilty and act very suspicious, prompting Nelson to investigate. Meanwhile, Marge thinks Homer is cheating on his diet.
Homer and Marge are in bed about to snuggle. Homer, however, is far too out of shape to do anything. This prompts a worried Marge to make a healthy dinner consisting mostly of vegetables. Marge decides to hire a formerly obese nutritionist named Betsy Bidwell to help with Homer's food addiction. Betsy tells Homer of a simple trick called "substitution": Whenever Homer feels like eating something unhealthy, explains Betsy, he should substitute it with a bell pepper. Homer begins eating the bell peppers everywhere, including at a baseball game, and at Moe's Tavern as a beer smoothie. After Betsy and Marge measure Homer's weight, a shocked Betsy informs Homer that instead of losing weight, he has gained seven pounds. Marge suspects that Homer may have cheated on his diet. While Bart and Lisa are watching television, Marge discovers a show called Sneakers, which purports to investigate people suspected of cheating on their partner. Marge decides to call the producers, with the intention of having them stalk Homer and see if he is cheating on his diet. Marge manages to reach Sneakers and makes an appointment to talk to the host, and the show decides to help her.
Bart and Lisa decide to tour Springfield Park. At the top of a mountain, the two discover Martin excavating for arrowheads. While Lisa joins him in excavating, a frustrated Bart decides to play a prank on Martin. He steals Sideshow Mel's bone (which causes his hair to fall down, revealing it to be long and straight), and partially buries it in the ground for Martin to find. Martin arrives and thinks it is a real historic bone, and Bart tugs on a string connected to the bone, which causes the bone to fly up and knock Martin in the head. A stunned Martin tumbles over the edge of the cliff, and falls onto a smaller ledge. Lisa, trying to help him, taps him with a stick, which causes Martin to fall off the ledge and to the very bottom. Shocked at what happened, they presume him to be dead. The two arrive home, where Kent Brockman reveals on the news that Chief Wiggum discovered that Martin was apparently dead and his body had vanished. On TV, Wiggum says that when Martin landed, he was eaten by a cougar, which is still coughing up his clothes. Although Bart wants to admit to what happened to Martin, Lisa, frightened of going to jail for being an accomplice, convinces Bart to deny they were responsible. Homer then abruptly leaves, claiming he is going to work at 4 o'clock on a Saturday. Marge calls Sneakers and tells them Homer left the house, prompting a nearby van full of Sneakers agents to stalk Homer. They follow him to a motel room, where he lustfully eats a stack of pancakes, even taking the pancakes into the shower. After briefly observing Homer through camera in the shower, the Sneakers host has had enough, and orders the cameras to be turned off.
At Springfield Elementary School, a memorial is held as a to tribute Martin. Bart begins to panic frantically, thinking someone would investigate. Sure enough, Nelson Muntz starts getting suspicious, questioning why Martin would be anywhere near a cliff, especially considering that Martin had a fear of heights. Nelson goes up to Mt. Springfield and discovers Sideshow Mel's bone on the string Bart pulled. Meanwhile, the Sneakers agents set Homer up to go to a restaurant for Marge to confront him about his diet cheating. Marge does so, but she realizes that the reality show people do not care about their marriage. She slaps the host instead of Homer and embraces Homer, stating that she'd "rather have a chubby hubby than a sexy exy."
Meanwhile, Bart and Lisa arrive at Martin's greenhouse, where they are spooked by a recording of Martin's voice. Lisa is so frightened that she blurts out what happened at the cliff, and Nelson arrives just in time to catch Lisa's confession on tape. Right after that, Martin appears, shocking Nelson as well as Bart and Lisa. Martin recounts how he survived the fall thanks to his underwear with an extra-durable wedgie-proof waistband. Lisa learns from the ordeal that underneath her innocent looking self is a dark twisted person.
The episode had 7.3 million viewers. It has received pretty positive reviews.
Richard Keller of TV Squad quoted "It was a pretty good episode". He went on to say "The addition of Nelson as an investigator to the conspiracy was an interesting move on the producers' part". The Homer sub-plot was "Entertaining and better than the rental car plot he had in The Debarted".