Ryou Kizuki, known as Ryo Inasa, journeys to her late mother's isolated island home seeking answers about her mother's eccentric life and origins, defying explicit warnings to avoid the island and her cousin Hotaruko. Upon arrival, Hotaruko and Ryou's lover, the servant Ruka, greet her.

Ryou quickly encounters disturbing events tied to a dark island cult devoted to Master Mizuno, a deity housed in puppet form. This cult lures women into ecstatic states before sacrificing them in bloody rituals. Ryou witnesses such a ritual, seeing the sacrificial murder of Misa, servant and lover to Kasumi (the priest's daughter).

Ryou uncovers devastating truths about her lineage. The cult's chief priest, Master Komoshi, reveals Hotaruko is not her cousin but her biological mother. Itsuki, believed to be Ryou's mother, is her grandmother, making Bungo (Itsuki's brother) her great uncle. Mizuno is identified as both Hotaruko's brother and Ryou's uncle, implying incest within the family, with further implications that Mizuno may be Ryou's father.

Driven by relationships, Ryou chooses to stay on the island with the severely wounded Hotaruko, ignoring both Ruka's pleas and Bungo's self-sacrifice meant to aid their escape. Later, under the influence of the cult's passion-inducing red flowers, Ryou consents to a sexual encounter with Mizuno. Ruka intervenes, killing Mizuno to protect Ryou. Ryou, descending into apparent madness, retaliates by killing Ruka.

The story concludes with Ryou suffering the cult's sacrificial fate. After the credits, it is revealed she has given birth to a child, implying Mizuno's rebirth and the perpetuation of the island's violent ritual cycle.