3 Double, Double, Boy in Trouble

The episode begins with Maggie, Homer and Bart at the Kwik-E Mart. Apu tries to get Homer to buy the last lottery ticket by claiming that the last ticket is always lucky. When Homer is about to take money out to buy the ticket, Bart attempts to jump off a shelf and land in Chief Wiggum's cart full of marshmallows. However, Wiggum moves the cart and Homer has to race over to catch his son, while Lenny walks up to the counter and buys that last ticket. Lenny wins $50,000, which makes Homer jealous. Apu decides to put it in his book, Tales of the Kwik-E Mart. At Moe's Tavern, Lenny announces he is going to spend his winnings on a giant party at the Woosterfield Hotel for all of his friends. The Simpson family cannot find Bart when it is time to leave, because he is upstairs in the attic with a water gun full of cat urine. When he is about to shoot it at Rod and Todd Flanders in a wagon below, Marge steps in and Bart accidentally sprays her with the urine, so Marge has to wear a mediocre "back up dress" to the party. Homer and Marge wonder why Bart cannot behave and decide he went bad in utero when pregnant Marge accidentally swallowed a small drop of champagne after Mayor Quimby christened a new Navy vessel, the U.S.S. Float-and-Shoot. The drop landed on a gestating Bart and turned him bad. At Lenny's party, Bart discovers that Lenny will give out vacuuming robots in gift bags. Bart activates all the dangerous settings on them and they attack the party guests. When everyone finds out that Bart is responsible, Marge takes away Bart's non-dice board game privileges, after Bart says she already took away his TV and video game privileges. In the bathroom, Bart meets Simon Woosterfield, a kid who is both Bart's exact look-alike and part of a billionaire family whose wealth even eclipses Mr. Burns'. They also meet an adult Bart double who has a wife that, to Bart's disgust, looks like a female version of Milhouse.

The boys decide to secretly switch places and live each other's lives for a while by trading their clothes in the bathroom. Bart likes his new life as a rich child until he meets his stepsiblings, Devan and Quenly, who put a spoon heated by a lighter in his mouth because Simon is blocking their full inheritance of the family fortune. Simon refuses to eat Marge's recipe of cooked noodles with root beer and Cheetos, so Homer eats it and chews with his mouth open. When Simon is sent to bed without supper after calling Homer a spu-monkey for spitting food on him, Marge gives him pizza with no crusts and tucks him in, which Simon says he can get used to. The next day, the Woosterfields hold a party for all of their rich friends. When Devan and Quenly lock him in the Woosterfield mausoleum telling him that the bodies turn to candy, Mr. Burns gets him out. He tells Bart he was once the youngest in a wealthy family and his siblings all died in varied ways (mostly related to eating poisoned baked potatoes) and that Bart is in danger from his own family. Bart realizes that Devan and Quenly want to murder Simon so they can take his share of the inheritance.

When Simon listens politely to Abe's boring and ridiculous stories, Lisa concludes that Simon is an imposter and Simon explains his story to the Simpsons, saying that Devan and Quenly are taking Bart to Aspen where they will try to kill him. Before the Simpsons get to him, Quenly pushes Bart down a hill for experienced skiers. When Devan says that they will split Simon's inheritance, Quenly offers Devan a baked potato (a reference to Mr. Burns' story from earlier) planning to clear her path of him to snare 100% of the fortune for herself. Homer manages to save Bart in time, and Simon is welcomed to his family again via a hot fudge sundae by his butler Chester. The episode ends with Bart being lovingly tucked into bed by Marge and contentedly saying "This is the life."