Yuka's husband passed away in a traffic accident, so she raised Kosuke alone. Kosuke is a writer, seemingly working on a piece about his feelings for his mother. His feelings for his mother go beyond filial love, but he always hides his true emotions. Yuka also feels the same, as Kosuke increasingly resembles his deceased father, causing her feelings towards her son to shift in a wrong direction. Both constantly masturbate while thinking about each other, each time intentionally leaving the door open for the other to discover. Despite this, they have not crossed the final boundary of ethics. However, on the night before Kosuke's wedding day, after confiding in his mother, Kosuke returns to his room to continue writing his novel but cannot restrain himself from masturbating while thinking about his mother. This time, Yuka does not just watch, she enters the room and wants to be with Kosuke. Both express their true feelings and gradually overcome the final boundary...