Simon Woosterfield is a rich boy who looks exactly like Bart (who described Simon as a "dorky version of himself"). They found out about this in a bathroom and, after listening to each other's problems, they decided to switch lives.
Simon's older half-brother and half-sister, Devan and Quenley, wanted to kill him to inherit his share of the Woosterfield family fortune. When he found out about Bart, Simon lived Bart's life the entire day, but he and Homer argued because of Homer's manners, and Simon was sent to "his" room without food.
However, he enjoyed Marge's attention and love when going to bed. Lisa suspected that he was not Bart. Simon tells them of Devan and Quenley trying to kill him at the ski trip at Aspen. After Simon's half-siblings almost kill Bart (and then try to kill each other thinking Simon is dead), they switch back to their regular lives again.
Homer asked Simon about his half-siblings wanting to kill him. He doesn't worry about that because they are now planning to kill each other. Then, upon seeing how Simon's family lives, Homer begs to come with Simon but the car drives off.
- Devon and Quenley are full-siblings to each other (meaning they have the same mom and dad)--as Simon explained to Bart, all three of them share the same dad, but Simon's mom is their stepmom. It's unknown who Devon and Quenley's mom is.