Masks is a complex novel dealing with the interrelations among a group of people in intellectual Japanese society in the 1950's.

The male protagonists are Tsuneo Ibuki, a professor, and his friend Toyoki Mikame, a doctor. Both men are in love with Yasuko Togano, a young widow whose husband Akio was killed by a snow avalanche on Mt. Fuji. Behind Yasuko stands the dominating presence of Akio's mother, Mieko. As the story progresses we learn that Akio had a twin sister, Harume, who is mentally challenged and has been kept in seclusion. We also find out that Akio and Harume were not the children of Mieko's husband but of another man who died at the front in World War II.

Mieko is an expert on the Heian period of Japanese history (794 to 1185 CE), on the subject of the masks used in Noh (a type of classical Japanese musical theatre), and on the literary work The Tale of Genji. A paper she has written on the characters in Genji forms a kind of focal point of the novel, since it expresses Mieko's fascination with the issue of spirit possession, specifically that exercised on others by the Lady Rojuko in the Genji story. Mieko herself begins to manipulate the male characters in Masks, encouraging both Ibuki and Mikame to pursue their interest in Yasuko, and in effect taking possession of their will, or attempting to do so. She seems to favor Mikame as the suitor for her daughter-in-law, but encourages Yasuko to lead Ibuki on. During a tryst between Yasuko and Ibuki, Mieko perpetrates a ruse in which Harume, the mentally challenged daughter, is substituted for Yasuko in the middle of the night. Ibuki unknowingly makes love to Harume and impregnates her. Mieko thus achieves her goal of perpetuating her family line, though doing this through manipulation and deception.

Though outwardly Masks deals with elements of Japanese literature and history and the attempt by a modern woman to recreate them, so to speak, in her own life, the subtext of the novel involves gender issues and the changing status of women in twentieth-century Japan and in the world overall. Mieko is a woman who was dealt with unfairly by her husband, who openly lived with another woman as his mistress. She had no recourse to justice in the rigid patriarchal society prior to the war, and in some sense her use, much later in life, of both her daughter-in-law and her daughter to control men is an attempt to vindicate herself. But, as the title indicates, each of the characters is wearing a kind of mask in unconsciously carrying out wishes that are external to them, imposed upon them from the outside. The novel as a whole is an intense meditation on the roles people play and the way destiny and the desires of others cause us to act in ways we ourselves don't fully understand.