After divorcing her husband, Mirei lives alone with her son, Yoshihiko, having lied to him that his father died. Yoshihiko believes her, unaware that he even has a younger brother. One day, her ex-husband suddenly visits Mirei's home, pleading for reconciliation. Though reluctant, Mirei is deeply moved when he tells her that her stepson, Haruhiko, wants to see her. That night, she begins a secret affair with Haruhiko behind Yoshihiko's back—leading to an inevitable spiral of incest, titty fuck, and forbidden 69 positions as emotions and desires collide.