Bart accidentally has the entire town emulating his actions, thanks to a feel-good counselor.
Homer comes across an advertisement in the newspaper for a free trampoline. He rushes to the address from the advertisement, where Krusty the Clown is giving it away--when Homer questions why Krusty's getting rid of it, the clown explains that he used it for various gymnastics routines (such as tumbling), but he's phasing out all the gymnastics in his act and replacing it with things like dirty limericks. Homer takes the trampoline home, and his kids are thrilled by it--Marge, however, is concerned about the potential dangers. But Homer brushes aside her worries; he has grand plans of turning their backyard into a theme park and plans to charge others a fee to use the trampoline. Inevitably, however, people start getting hurt, and Homer finally takes Marge's advice to get rid of the trampoline. After failing at his various attempts to do so, Bart steps in to help, by chaining the trampoline to a pole using a bike lock and waiting for Snake Jailbird to steal it (who uses it as a bed).
After Homer and Marge go to bed later that night, Homer admits to Marge that while she was right in that getting the trampoline was ultimately a mistake but also points out that he is at least willing to go out and try new activities, whereas Marge is just a boring nag who never does anything new. Marge, of course, disagrees with what Homer says, but after asking what Bart and Lisa think, she discovers that they agree with their dad's assessment about he, and even bring up times that prove that all Marge really does is ruin everyone's fun by nagging at them. Marge becomes uncomfortable and offended after discovering that people see her that way and goes to the Spinster City Apartments (almost hitting Ned Flanders with her car on the way out). While Marge is at Spinster City, Patty and Selma introduce Marge to an infomercial featuring self-help guru, Brad Goodman, who can supposedly help people like Marge with their "chronic nagging."
After Marge makes Homer watch a Brad Goodman video with her, she becomes more tolerant and the two start getting along better. After seeing how out of control Bart is, the family goes to see Goodman's live lecture in the hopes that it will change him. Bart interrupts the lecture, but Brad Goodman encourages the town to follow Bart's spontaneous attitude. Soon, the whole town starts acting like Bart, who at first enjoys things, but eventually becomes depressed by it. Lisa explains that it is because he has lost his unique identity as a rebel with everyone else in town behaving just like him.
The citizens of Springfield hold a "Do What You Feel Festival" where everyone does what they feel. Unfortunately, this results in workers not doing their jobs, culminating in an ungreased Ferris wheel coming off its hinges and crashing into the zoo whereupon the animals escape and run amuck through town. People begin having great big fights by arguing with one another and beating up each other until they all choose to blame Bart for the mess (though Rev. Lovejoy pointed out that Brad Goodman was the one who inspired the town to act like Bart in the first place). They form a mob to attack him, but Homer rescues him in a parade float. The crowd almost immediately gives up the chase, despite the float's very slow speed, and goes to the old mill to get some cider.
Back home, the Simpsons discuss what they have learned. Homer feels that Bart should have been a better role model, but Marge comes to Bart's defense by claiming that self-improvement is best left to people who live in big cities. But Lisa claims that the actual lesson is that self-improvement can be accomplished, but through hard work and not a quick fix. But Homer concludes that everyone's fine the way they are. With that sorted out, the family watches McGarnagle, a show about a cop who solves crimes in his spare time.