After Lisa's new pet guinea pig destroys the Simpsons' living room art, Marge falls in love with a beautiful painting at the Van Houten's yard sale, which Homer purchases for $20. Marge dislikes the frame and removes it, revealing a previously invisible signature "Johan Oldenveldt". Lisa researches Oldenveldt and discovers he is of some fame, so Homer and Marge have the painting appraised in a loal auction house. The appraiser judges the painting to be valuable, and estimates it could auction for $80,000-$100,000.
Marge thinks the proceeds from the painting's sale should be shared with their friends, the Van Houtens, but Homer thinks for the Simpsons' future financial security they should keep the money for themselves. He eventually persuades Marge that the Van Houtens would dislike knowing they had missed the painting's value and sold it cheaply, but Milhouse, who is in Bart's treehouse, overhears the conversation and eventually tells his parents.
When the Van Houtens publicly shame the Simpsons for their secrecy, the town divides over whether or not the Simpsons' decision was the right one, with two crowds of opinion gathering outside the auction house as the painting is brought to auction. Shortly after the bidding begins, Dawn, an ex-lover of Kirk's, whom he had dated during a prior separation from Luann, arrives and claims he took the painting from her. The auction is put on hold until the painting's ownership is determined. Kirk tells Homer he bought the painting in Isla Verde, after which Dawn dumped Kirk. Homer takes Lisa to the café from which the painting was purchased to corroborate Kirk's story and hence confirm his own ownership of it.
The café owner confirms she sold Kirk the painting. As Homer celebrates, a man in the café named Klaus Ziegler decries Homer's ignorance of art, saying, "You don't even know who painted that picture". It transpires he is the true painter, and forged Oldenveldt's style and signature so effectively the auction house was fooled. Ziegler admits he has fooled art galleries worldwide. Although Lisa objects to his practices, he ultimately convinces her the beauty of art depends only on how it moves rather than whether its authorship is honestly presented. Homer pays Klaus to draw them three new pieces of art: a family portrait for the Van Houtens, and a new boat painting for the Simpsons' living room... as well as a loud jukebox picture.But suddenly during the credits, it shows a documentery of how Strupo has affected people's lives, then cuts back to the credits.