Natsumi lives with her husband, who remarried, and his son Yoshio. With no sexual relationship with her husband, Natsumi struggles with her mature, unfulfilled desires. Sensing her inner turmoil, Yoshio takes advantage and turns her into his sex slave. Every time her husband leaves for work, the stepmother and stepson's depraved affair begins. Soon, Yoshio's friends join in, treating Natsumi's body as their personal toy, taking her one after another. Though her mind resists, her body gradually succumbs to the overwhelming pleasure.