Phoenix Jackson is an African American grandmother who walks to Natchez to fetch some medicine for her grandson.
The journey is a long one, and Phoenix is frail. However, she braves the dangers regularly so that her grandson can get some relief from his suffering. According to the story, Phoenix's grandson swallowed some lye two or three years ago. The incident continues to cause the boy great suffering: his throat becomes constricted on a regular basis. When that happens, it is difficult for him to breathe. The medicine that Phoenix gets eases her grandson's suffering.
The doctor has told his nurses that Phoenix can have the medicine for free if she can make the journey to get it. Being poor, Phoenix has to make the journey on foot. However, she doesn't mind, because her grandson is the only family she has. We can tell that Phoenix loves her grandson very much. The journey is a difficult one for her, and yet she willingly makes it. During her interaction with the nurse, Phoenix proudly mentions her grandson's patient and long-suffering character.
Essentially, the story is about the love of a grandmother for her grandson and how she willingly endures challenges for his sake.