Yuriko had a physical relationship with her son, Reiji. One day, she goes on a trip with her sister, Junko, and her son, Masao. At the condominium where they stay, Yuriko and Reiji plot to draw Junko and Masao into this forbidden relationship. Yuriko seduces Masao with her proud body. Reiji attacks Junko, and the relationship develops into an unprecedented act of incest.