The Third Wife is set in the 19th century and follows a 14-year-old girl who becomes the third wife to a landowner in rural northern Vietnam.
14-year-old May is married off as the third wife to a middle-aged landowner of a rural village where silk is harvested. She soon finds that, having given birth to a son, the first wife exerts greater influence in the family than the second, who has only had three daughters, and that the only way to gain security and independence is to give birth to a male child.
Her husband's wives are welcoming to her and when she expresses that she finds sex painful they urge her to experiment sexually with herself in order to understand her own desires.
May conceives a child. While this brings further intimacy between May and her husband she continues to find him unappealing and rejects his sexual advances. She discovers that Xuan, the second wife, is having an affair with the landowner's son from mistress Ha, his first wife. Due to May rejecting her husband, he returns to his first wife and conceives a child with her as well. May prays her own child is a son who will secure her position within the family.
Mistress Ha has a miscarriage and May blames her prayer for a son as the reason for it. She is reassured by Xuan who tells her that throughout her pregnancies she too prayed for sons. May realizes that she has burgeoning feelings for Xuan and kisses her. Xuan rejects May's advances, brushing it off as a symptom of her pregnancy and telling her she loves her like a daughter. May gives birth to a girl.
Mistress Ha's son is of marrying age and his parents conceive of a match for him. In love with Xuan, he tries to refuse marriage, threatening suicide. He is nevertheless pressured to marry a young girl around May's age. Son is disgusted with the marriage and refuses to touch his new bride. After begging his father to annul the marriage his father attempts to do so, but is rejected by the bride's father. Ashamed the young bride commits suicide.
May contemplates poisoning her daughter with a plant she saw being used to euthanize the family's livestock. The film ends with one of Xuan's daughters, who had once expressed desire to become a man and have many wives, cutting her hair with a pair of scissors.