For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Boy Willie and Lymon arrive from Mississippi with a truck load of watermelons to sell. However, Boy Willie's real goal is to sell the family's historic piano, so that he can buy Sutter's land. His sister, Berniece, is dead-set against this. Not long after Boy Willie arrives, Berniece see's Sutter's ghost. She blames Boy Willie for the ghost's presence. The two siblings fight about the ghost and the piano for most of the act. It all comes to a head, when Berniece completely loses it and starts hitting Boy Willie. At this peak of tension, the ghost makes another ghastly appearance, but this time to Berniece's daughter, Maretha.
Act II
The bickering over the piano continues, and Sutter's ghost continues to haunt the house. Berniece asks her preacher boyfriend, Avery, to bless the house to get rid of Sutter's ghost. Doaker catches Boy Willie and Lymon trying to take the piano from the house; he refuses to let them do it unless Berniece says it's OK. The piano is too heavy for the two men to lift anyway (and may be held in place by Sutter's ghost). The act peaks when Boy Willie declares that, as soon he gets wheels and rope, he's going to move the piano no matter what.
Act III
Everything explodes in the third act. When Boy Willie and Lymon try to move the piano again, Berniece threatens Boy Willie with a gun. Avery shows up to bless the house, and the ghost of Sutter attacks. Boy Willie tries to wrestle the ghost, but seems to be fighting a losing battle. The play reaches its thrilling climax when Berniece plays on the piano summoning the spirits of her ancestors and banishing Sutter's ghost. Boy Willie gives up his plan to sell the piano, but tells his sister that she better keep playing on it, or he and Sutter's ghost might just be back.