"Double, Double, Boy in Trouble" is the third episode of Season 20. (Originally was supposed to be a episode of the Season 19.)
Bart meets his doppelganger, Simon Woosterfield, a member of the richest family in Springfield and the two plan to switch places. Simon adjusts nicely to his new digs at the Simpson home, while Bart discovers that his new siblings are plotting his demise so they can gain possession of his inheritance.
At the Kwik-E-Mart, Homer hurries to get the last lottery ticket, but is distracted by Bart dancing on the shelves, then attempting a cannonball onto the floor. While Homer tries to save Bart, Lenny buys the lottery ticket and wins $50,000. A now gleeful Lenny invites a very sullen Homer to a rich, posh party at the expensive Woosterfield Hotel. While going to the party, Bart unintentionally sprays Marge's new dress with Snowball II's urine (which was meant for Rod and Todd Flanders), nettling her and Homer. While Homer tells Lenny he is grateful for his success, Bart is in the back, concocting a plan to destroy the party. He opens all the gift bags and releases the Vac-U-Bots, which run wild and cause mayhem, destroying everything. The police catch Bart; Marge, felling outraged, says she will confiscate his TV his video game privileges and all of Bart's non-dice board games which she already did. Bart is displeased with his family and heads to the restroom, where he meets Simon Woosterfield, a rich boy who turns out to be Bart's exact look-alike, they also meet a man who looks like an older version of them whose married to a woman who looks like a female version of Milhouse but he leaves minutes later. Bart learns that Simon dislikes his rich and pompous lifestyle; in order to satisfy each other, the boys plan to switch places, Simon going with the Simpsons, and Bart going with the Woosterfields.
After informing each other on basic information about each other's family, the two part; Bart finds he truly enjoys his new rich home, which appeals to him immediately (particularly Joe Montana, who is hanging around the estate). Simon, meanwhile, isn't used to the Simpsons's simple lifestyle; Lisa, the most inquisitive, begins to suspect that Bart (Simon) isn't who he really is. Simon frequently argues with Homer, but upon going to bed, he soon begins to enjoy Marge's loving attention. Meanwhile, Bart meets Devan and Quenley, his new half-brother and sister. The two of them smugly bully Simon (Bart) every chance they get. The next day, the Woosterfields host a magnificent party for the rich; upon meeting Mr. Burns and Smithers, Bart asks why Devan and Quenley always bully him more than they should. Mr. Burns tells all about his childhood growing up as the youngest of a large wealthy family, and that since his other siblings passed away under 'suspicious circumstances', he inherited the family's entire wealth. Bart realizes that the two may very well be plotting his demise, in order to inherit the entire Woosterfield fortune. Bart becomes irritated when he realizes about being set up by Simon.
Lisa quickly realizes that Simon is not Bart, and tries to figure out what they are up to. Simon finally admits to the entire Simpson family that Bart is in real danger. Devan and Quinley take Bart on a ski trip, hoping to kill him at the ski lodge. While trying to rescue Bart, Homer falls down and turns into a giant snowball, rescuing him. Bart is still offended with Simon, but Simon tells him that he will return to his own family; when Bart asks what he will do to prevent the murderous attacks Devan and Quinley are planning to make, Simon tells Bart not to worry about it. In truth, the two are already planning to dispose of one another. Upon returning home, Marge tucks Bart into bed, tickles his foot making Bart giggle and kisses him goodnight. After mulling it over, Bart chooses that "this is the life" and falls asleep happily.