After Bart sneaks away from a field trip to the Box Factory, he finds himself working as Krusty's assistant. Krusty lets Bart act out a scene on one of his gags, but Bart messes up the line and trashes the set he then says, "I didn't do it." The phrase is an instant success and leads Bart to his fifteen minutes of fame.
When Bart's class goes on a field trip, what Bart is expecting to be a day full of fun is soon dashed when Mrs Krabappel bitterly guesses Principal Skinner is taking them to the box factory (again). As they drive most of the students and the teacher stare with their faces against the windows at other classes also on field trips but to much more exciting places before pulling up at the box factory. The tour is something which everyone except Skinner and Martin find to be exaggeratedly dull. This includes Bart, who get bored with the tour and sneakily escapes from the class to go to the Channel 6 TV studio nearby, where he wanders by a beach scene and Bumblebee Man. With his absence, Skinner calls Homer who arrives, thinking that Bart was killed in one of the giant box-making machines.
Meanwhile, Bart meets up with Krusty, who is angry that he does not have his Danish (Bart ate the last Danish, which was meant to be for Krusty). Because of this, Bart steals a Danish from Kent Brockman and gives it to Krusty, who then becomes grateful and asks him to become his assistant (Krusty has forgotten all the other times Bart has helped him out, from saving his career to renewing Krusty's relations with his father). When Bart returns home, Homer is relieved that he does not have to tell Marge that Bart was killed in a box accident. Afterwards, Bart continues his work as Krusty's assistant, though soon he gets tired, in that the cast members do not treat him well, and he does not get credited for his work helping the show. However, before Bart decides to quit, Krusty offers to let him say one line in a sketch. Bart messes up his lines and destroys all of the props on the stage. When the crowd's focus comes on him, he tries to get out of it, saying, "I didn't do it", which causes the audience to laugh and applaud. Seeing this, Krusty immediately uses Bart and his "I didn't do it" catchphrase in later sketches, and eventually creates a franchise out of it.
As Bart continues with his "I didn't do it" catch phrase, he begins to fear that the fad will wear off, so he tries to act intelligent (as during his interview on Conan O'Brien's show). Eventually Bart's catchphrase loses its humor and Bart loses his fame.
Marge gives Bart a box of items she kept during his stint, and gives it to him to help him remember his time there. When Homer accidentally breaks the Simpsons' lamp, it sets off a number of memorable catchphrases:
Everyone looks at Lisa expectantly. However, Lisa isn't amused and says "If anyone wants me, I'll be in my room." As she goes upstairs, Homer asks, "What kind of catchphrase is that?" Lisa is the only member of the immediate family without a catchphrase.