Yuji introduces a mysterious dice game to his mother, claiming it is a way to bring people closer. As they play, the game exerts a supernatural influence, compelling them to engage in increasingly explicit and forbidden sexual acts. Despite their initial hesitation, the two eventually succumb to the pleasure and form a secret, intimate bond as lovers.
After Yuji brings home a mysterious dice game and asks his sister to explain an obscene word on one of the cards, the resulting tension leads to an unexpected sexual encounter. The two siblings confess their hidden feelings for each other and begin a secret relationship, leaving their mother wondering why they have suddenly stopped bickering.