Momoyo, daughter of a wealthy household, is observed through a ceiling opening by the servant Sanshiro, who desires her. Her arranged engagement to Yoichiro, returning from abroad to formalize their union, initiates significant changes in her behavior and circumstances. Following Yoichiro's return, Momoyo engages in explicit sadomasochism with him, extending to multiple members of the household. These acts escalate beyond personal gratification into ritualistic practices intended to summon a demonic entity. Her participation shifts from apparent willingness to victimization under supernatural influence. Her arc centers on corruption and loss of autonomy as the rituals orchestrated by Yoichiro progressively ensnare her, her sister, and household staff in a spreading "plague of lasciviousness." Sanshiro interprets these transformations as demonic possession, positioning Momoyo as central to the impending awakening of a demon. Her role culminates in Sanshiro's intervention during a confrontation to prevent her complete destruction. Momoyo's fate remains intrinsically tied to the supernatural stakes of the ritual, emphasizing her status as a target of both human obsession and otherworldly designs.