After Lisa's new pet guinea pig destroys the Simpsons' living room art, Marge falls in love with a beautiful painting at the Van Houtens' yard sale, which Homer purchases for $20. Marge removes the frame and finds that the painting bears the signature of Johan Oldenveldt, an artist of some renown. An art appraiser estimates that it could bring between $80,000 and $100,000 at auction. Marge wants to share the sale proceeds with the Van Houtens, but Homer disagrees, saying that the Simpsons should look after their own financial security first and keep the sale a secret from the Van Houtens. Milhouse eavesdrops on the conversation from Bart's treehouse and tells his parents. The Van Houtens publicly shame the Simpsons for their secrecy, prompting a town-wide division of opinion.
As the auction begins, Dawn, a former lover of Kirk's, arrives and claims that he took the painting from her. The auction is suspended until its ownership can be established. Kirk tells Homer that he bought the painting on the island of Isla Verde, home to an artists' community. Homer and Lisa travel to the café at which Kirk made the purchase in order to corroborate Kirk's account and the legality of Homer's purchase. The café owner confirms that she sold the painting to Kirk, but as Homer starts to celebrate, one of the customers interrupts. This man is Klaus Ziegler, an art forger who created the painting; he has fooled art galleries around the world by flawlessly imitating other painters' techniques. Although Lisa objects to Ziegler's practices, he convinces her that his forgeries have brought pleasure to the people who see them. Homer pays him to create three new paintings: a family portrait for the Van Houtens, a new sailboat picture for the Simpsons' living room, and a garish picture of a jukebox for Homer.
The episode ends with a brief documentary (narrated by Ziegler) on the history of Strupo, a foul-smelling, hallucinogenic, highly addictive alcoholic beverage brewed on Isla Verde.