Bart Mangled Banner is the twenty first episode of Season 15.
A photo showing Bart accidentally "mooning" the USA flag makes the Simpsons the most hated and unpopular family in town.
Homer and Marge take the kids to get their vaccination shots. Just before Dr. Hibbert is about to inject Bart, he escapes. After a chase through town, Hibbert finally outsmarts Bart by having Barney (and briefly Moe) impersonate him and the real Hibbert injects Bart. However, the shot causes Bart's ears to swell shut, making him temporarily deaf.
While at the Springfield Elementary School donkey basketball game, Bart teases a donkey with a carrot, unaware that the school is reciting the National Anthem. After he places the carrot in his shorts, the donkey takes it out and rips off Bart's pants (just like his famous punchline "Eat my shorts!"), thus exposing his bare bum. While Bart is bent over to cover his private part with his sweatshirt, the USA flag is put up behind him without knowing and a photo is taken, which results in the crowd assuming that Bart is mooning the USA flag. Marge and Homer try to explain to Skinner that Bart didn't mean for what he was doing because he was deaf at that time ; however, because of Bart's history of pranks Skinner doesn't believe it. Shortly afterwards, The Springfield Shopper takes the photo page and completely turns it around, making it seem as if Bart had mooned the flag on purpose which was titled "U-Ass-A!". Soon, everyone in Springfield becomes angry at the Simpsons family.
Bart and his family do not hate USA but when the family tries to clear up this misunderstanding, it is to no avail. Later, the Simpsons are asked to appear on a talk show and tell their side of the story. However, instead of listening to their side of the story as promised the host instead asks, "What part of USA do you hate most?" (an example of the fallacy of many questions), also turning the question on Homer and Marge, the later of whom answers "Well America is not perfect". When the host asks if this is why the Simpsons hate Americans, Marge snaps, saying that if he is referring to loud-mouthed talk show hosts, which she notes "everyone seems to be in this country" (referring to the fact that questions such as that one are the only forms of discussion in USA), then she does hate Americans.
This sentiment leaves everybody shocked, with even Bart, Maggie (who is too young to understand) and Homer (who is eating a sandwich) left taken aback by what Marge said. When the host expresses surprise that Marge has not been run out of town she replies that she is well-liked in Springfield, prompting him to declare that Springfield hates America. As a result, the USA turns its back on Springfield, even going so far to declare December 25th "We hate Springfield Day". Mayor Quimby frantically decides to change the name of Springfield to "Liberty-field". Everything in town is quickly patronised; the traffic light colors are changed to red, white, and blue, and mostly big things costs $15 and small things cost 50c. While at church, Lisa speaks her opinion about patriotism, and the Simpsons are taken into custody, in violation of the "President Best Act".
The family are taken to the "Ronald Reagan Reeducation Center", which houses Michael Moore, the Dixie Chicks, Elmo (who accidentally went to the wrong fundraiser), and Bill Clinton. With some help from the last-registered Democrat, the Simpsons escape the jail, but realise that the re-education center is actually Alcatraz Jail. While they are swimming to land, they are picked up by a French freighter and are brought to France. They are well adjusted, but still miss USA, mainly because it's where all their stuff is. Then they move back to USA dressed as steam-fashioned immigrants from Europe, where Homer speaks of plans of integration back home to Springfield.