To take care of her poor and weak siblings, Wu marries Jia Hongju, who is old and often sick. Jia has two married younger brothers, and their wives often scorn and abuse Wu because of her humble origin. Nan, a newly appointed governor, stays in the post house next to Jia’s house. He happens to see a woman who intends to drown herself in a well and hastens to stop her; the woman turns out to be Wu. Full of sympathy for her, Nan writes a poem on a piece of paper, which is unexpectedly snatched by a swallow and taken into Wu’s room.
Before he goes to the capital to buy himself an official rank, Jia’s third younger brother entrusts Wu with the key to the storehouse. Coveting the property, Jia’s second younger brother and his wife murder the old man and blame the crime on Wu, offering the piece of paper as evidence of her treachery. The case is tried by none other than Nan, who is confounded by the sham evidence. Wu commits suicide to show her innocence. Before she dies, she hands over the golden key to her sister Erman. Nan’s father has such a strong sense of justice that he orders his son to find out the truth in ten days or he will suffer punishment. Fortunately, Nan is helped by Erman, who, seeing a swallow taking the paper away, solves the mystery.Wu’s innocence is proven and Nan is exempted from punishment. The opera comes to a happy ending.