Yukino Shirakawa warmly welcomes Daigo, her husband's younger brother, who has come to Tokyo to prepare for his entrance exams. For Yukino, who grew up as an only child, living with a brother-in-law felt like a dream come true. But when her husband is away, Daigo's demeanor changes completely, and he forcefully takes her. Torn between loyalty to her husband and overwhelming guilt, Yukino finds Daigo's demands growing ever more intense. Gradually, a new emotion—different from her feelings for her husband—begins to stir within her.