The events of "Why I Live at the P.O." are set in motion when Stella-Rondo, the narrator's sister, returns to the family home in China Grove, Mississippi. The narrator, known as Sister, claims that up to this point she has gotten along well with her mother, uncle and grandfather. Sister has a competitive and contentious relationship with Stella-Rondo, and her return sets off a chain of petty family arguments that serve to explain why Sister has moved out and now lives at the local post office.

It is the Fourth of July. Stella Rondo has left her husband, Mr. Whitaker, a man Sister had once dated, and brings home a daughter, a two-year-old child named Shirley T. The family did not know of Shirley T.'s existence and her age suggests that she was conceived before the marriage took place. Stella-Rondo explains that Shirley T...