Rika, a mother who has raised her son Mamoru by herself. Recently, she seems to be in a romantic relationship with a coworker of hers, and Mamoru, the mother's father, is not happy about it. This feeling is so strong that before Mamoru knows it, he sneaks into her bed and has sex with her at night... One day, Rika decided to bring her lover to her house. Mamoru leaves the room, expressing her discomfort, but when he returns to the room, he finds Rika entangled with her lover.