Bart goes on a class field trip to a box factory. He gets bored with the tour and escapes from the class to go to the Channel 6 TV studio nearby, where he encounters Krusty the Clown. Krusty is angry that his assistant has failed to get him a Danish, as Bart had eaten it, and fires his assistant on the spot. Bart steals a Danish from Kent Brockman and gives it to Krusty, who becomes grateful and asks him to become his new assistant. Bart soon gets disillusioned with being Krusty's assistant because the cast members do not treat him well and he does not receive any credit for his work. However, before Bart decides to quit, Krusty offers to let him say one line in a sketch when Sideshow Mel is unavailable thanks to Bart giving him a sandwich with cheese which triggers his lactose intolerance. Bart messes up his lines and stumbles, accidentally destroying all of the props on the stage. With the crowd and cameras focused on him, he exclaims "I didn't do it", which causes the audience to erupt with laughter. Seeing Bart's popularity, Krusty uses him and his "I didn't do it" catchphrase in later sketches, and eventually creates a franchise out of it.
Bart and his catchphrase continue to be popular, while Krusty creates a wide array of merchandise featuring Bart's likeness. Bart becomes sick of his fame and begins to fear that the fad will wear off, so he tries to expand his act and personality during an interview on Conan O'Brien's late-night talk show, to no avail. Bart decides to stop performing, but Marge convinces him that he should continue because he makes people happy. Bart arrives at Krusty's show and enthusiastically delivers his line, but the audience is bored with the line and does not react. With fans no longer interested in Bart, Krusty ditches him.
Marge gives Bart a box of items she kept during his stint as a celebrity to help him remember this time of his life. Lisa says that she is glad that "Bart can go back to just being himself, instead of a one-dimensional character with a silly catchphrase." The Simpson family — along with Barney Gumble, Mr. Burns, Ned Flanders, and Nelson Muntz — then recite their respective catchphrases, prompting an unamused Lisa to go to her room.