After attacking Lisa with spitballs, Bart finds out his last baby tooth is loose. After some failed attempts in getting it out, Bart's tooth comes out when Marge forces a drawer open. Bart puts his tooth under his pillow for the Tooth Fairy and in the morning, he finds a note saying the Tooth Fairy made a donation in his name to the United Way. He soon begins to realize that he is not a child anymore. After deciding that his childhood is over, Bart puts his toys into a small boat and sets it on fire, a proper "Viking funeral".

Lisa tells Bart that she writes things when she feels depressed. Bart does so and begins writing things on his T-Shirts that become popular. When Milhouse wants one of his shirts, Bart decides to go into business of making T-Shirts. His business is a success until the police confiscate his shirts because he has no license. Bart takes his shirts to a retailer's show in order to get a license, but his display is destroyed by Krusty the Clown's massive stand. While leaving, Bart is run over by Goose Gladwell, a Willy Wonka-type salesman who sells weird items. He looks at Bart's shirts and decides to sell them in most of his 20 stores which are in 30 states. The shirts are fast sellers and Goose gives Bart enough money to support the Simpson family income. The family structure is soon reversed when Homer gets fired by Mr. Burns (thanks to wearing one of Bart's T-shirts) and asks Bart for $200 to pay the $100 bill for their dinner in a restaurant and for breaking some bathroom fixtures there.

While watching a documentary about lions by Declan Desmond (voiced by Eric Idle), Homer decides to nurture Lisa after thinking Bart has replaced him. They quickly bond as Homer plays Malibu Stacey with her (while simultaneously managing to attack Ned Flanders). In Lisa's room, Homer sees her entry for the science fair, which is a history of nuclear physics and a scale model of the first nuclear reactor. However, Martin shows them his project, a childlike robot. With Lisa sure to lose, Homer decides to help her by stealing some plutonium from the power plant and building a small working Class II plutonium fission reactor. After showing it to Lisa, she is shocked and asks Marge to tell Homer to get rid of it. At Goose's store, Bart learns that Goose sold the rights to The Walt Disney Company to make his shirts into movies, but Bart will get nothing.