After attacking Lisa with spitballs, Bart finds out his last baby tooth is loose. After some failed attempts in pulling it, 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 of America. 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 with marker pens. They become popular around town, and when Milhouse wants one, Bart uses this as a business opportunity and draws on more of his shirts with pens, which he charges people to buy at a stand outside his house. The shirts are a success until the police confiscate them 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, which sells Itchy & Scratchy T-shirts. 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 British film producer Declan Desmond, 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 named CHUM. 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 horrified and alerts Marge to the danger, who tells Homer to get rid of the "irradiating whatsit". 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. He runs into Homer after leaving Goose and this angers Homer. In Goose's shop, Homer threatens to detonate his nuclear reactor and destroy the whole tri-city area if Goose does not give Bart what he deserves. Goose does so and Homer uses his leverage to get himself some novelty items as well. Bart thanks Homer for straightening things out.