Tsubomi lives with her stepfather and stepbrother. One day, her stepbrother suddenly declares that he wants a younger brother, and the foolish father takes it as permission to pursue Tsubomi, despite her not being his biological daughter. Soon, the stepbrother joins in, saying, "It's okay even if we're not real siblings," unwittingly becoming complicit in his father's twisted scheme. The darkness within this so-called family grows deeper.