Yuriko has been engaged in a forbidden relationship with her son, Reiji. One day, the two of them join Yuriko's sister, Junko, and her son, Masao, on a trip. At their condominium accommodation, Yuriko and Reiji decide to involve Junko and Masao in their secret. Yuriko uses her proud, voluptuous body to seduce Masao, while Reiji makes a sudden move on Junko. What follows is an unprecedented descent into extreme incest, unfolding in a shocking series of events.