Over one hundred years ago, the Charles family was owned by the Sutters of Mississippi. When Papa Boy Willie's wife and son were sold to a neighboring farm for a piano, Boy Willie used his skills as a master woodworker to carve their likenesses into the piano - as well as a pictorial representation of their family history, from Africa to the present.
Decades later, the descendants of that family - the brothers Wining Boy, Doaker, and Boy Charles - decided that the piano could not remain in the Sutters' possession. It was the soul of the family. They succeeded in stealing the piano, but Sutter caught up with Boy Charles in a Yellow Dog train car, and burned him and four hobos alive. These five became the Ghosts of the Yellow Dog, avenging themselves over time against the white bullies of the area.
The piano became the property of Mama Ola, Boy Charles' wife, who expended her sorrow on it. When she passed away, the piano belonged to her children, Berniece and Boy Willie, although for a time Wining Boy carried it around from town to town as a traveling musician. It currently resides in Berniece's living room, although she refuses to play it.
The action of the play revolves around Boy Willie's visit to Pittsburgh with the intention of selling the piano. He and his friend Lymon have come up north with a truckful of watermelons to sell. With his half of the melon sales and his half of the piano, as well as savings, Boy Willie will be able to buy Sutter's land - the elder Sutter having been recently killed in an accident that some attributed to the Ghosts of Yellow Dog.
But Berniece refuses to sell the piano. Her sense of identity is closely tied up in the artifact and its history, so although she won't play it, she also won't let it go. The siblings fight over this issue for the entire play, occasionally mediated by Doaker and Wining Boy.
Meanwhile, Berniece's personal life is similarly challenged. The preacher Avery wishes to marry her, but she isn't interested in being with a man at the moment. However, late one night a lovelorn Lymon manages to crack through some of her defenses. This encounter also causes Lymon to doubt his role in Boy Willie's scheme to remove the piano, and their last attempt to move it is thwarted by a combination of Lymon's doubt and the work of the ghost of Sutter, who has been haunting the household.
In a final confrontation, Boy Willie attacks this ghost, while Avery attempts to exorcise the house. Boy Willie is losing the fight with the spectre when Berniece performs her own exorcism - she goes to the piano and finally plays it, summoning the aid of the spirits of her ancestors.
Sutter's ghost flees and, finally convinced of the value of the piano - and of Berniece's renewed commitment to keeping its memory a living thing - Boy Willie leaves as well.