A number of burglaries by a cat burglar take place in Springfield, even striking the Simpsons' house. Among the stolen items include Lisa's saxophone, Marge's pearl necklace, Bart's stamp collection, and the handheld television. In response, the residents of Springfield arm themselves and install security devices. A neighborhood watch group is formed, and the members elect Homer as their leader. The group patrols the streets, but its members only end up violating laws further than catching criminals, turning into vigilantes. When Homer is interviewed on news anchor Kent Brockman's Smartline, the cat burglar calls the show and informs Homer that he will steal the world's largest cubic zirconia from the Springfield museum.
Homer and his group begin guarding the museum. Grampa and his friends from the retirement home volunteer to help, but Homer tells them to leave because they are too old. A few minutes later, Homer sees a group of teenagers drinking beer. He leaves his post to intervene and catch them, but immediately ends up getting drunk with them instead. While Homer's guard is down, the cat burglar sneaks past and steals the zirconia. Homer is blamed and pelted with vegetables by the unforgiving townspeople. Later that day, Grampa stops by the Simpsons' house and tells everybody that he knows who the cat burglar is: a resident in the local retirement home named Molloy.
Homer captures Molloy at the retirement home, and the surprisingly amiable cat burglar returns the objects he stole. Chief Wiggum arrests him and he is imprisoned. While in his cell at the police station, Molloy casually mentions that he assumes Homer and the cops probably want to know where he hid all of his loot. This piques their interest, and Molloy tells them the stash is hidden under a giant "T" somewhere in Springfield. They all rush out of the station hoping to get the treasure for themselves. In a matter of minutes the entire town hears of the existence of Molloy's stash and almost everybody in Springfield is racing to get there first. After the residents get to the site and dig, they finally discover a box with a note inside. The note tells them that there is no treasure and that while they have been searching, Molloy has escaped from his cell. However, several citizens continue to dig, convinced that a real treasure must be buried deeper.