Ryo Kizuki, also known as Ryo Inasa, journeys to her mother's ancestral island after her mother's sudden death, desperate to understand her mother's strange behavior and origins. Defying explicit maternal warnings to avoid the island and shun Hotaruko, Ryo pursues answers about her family's unresolved past. Upon arrival, she encounters her cousin Hotaruko and Hotaruko's servant Ruka. Ryo forms a close bond with Ruka, including sexual intimacy, while becoming drawn into the island's dark cult rituals. These rites involve luring women for sexual gratification before their brutal sacrifice to sustain the god-like entity Master Mizuno.

Ryo witnesses horrific events, including the sacrificial murder of Misa during a ritual. Priest Master Komoshi sexually stimulates Misa before cult member Kasumi impales her. These acts trigger traumatic memories of Ryo's mother exhibiting similar depraved behavior, influenced by the island's red flowers—blooms nourished by sacrificial blood that induce uncontrollable lust. Master Komoshi reveals critical truths: Hotaruko is Ryo's biological mother, not her cousin, and Itsuki—killed by mannequins earlier—is her grandmother. This implies Master Mizuno is Ryo's uncle and potentially her father, given Komoshi's assertion that Ryo exists because Itsuki gave birth to twins Hotaruko and Mizuno.

As events intensify, Ryo chooses to stay with the severely wounded Hotaruko after Hotaruko is attacked by Bungo—revealed as Ryo's great-uncle—ignoring Ruka's desperate pleas to flee. During the climax, Ryo consents to sex with a puppet containing Mizuno's soul. After Ruka kills Mizuno to save Ryo, Ryo immediately murders Ruka in a sudden psychotic break. The story concludes with Ryo suffering the same sacrificial fate as previous victims, implied to be orchestrated by Mizuno. A post-credits scene reveals Ryo is pregnant, suggesting Mizuno's rebirth and the continuation of the ritualistic violence cycle. Throughout her ordeal, Ryo's psychological stability fractures, transforming her from an investigator of family secrets into a vessel for the island's perpetuated horrors.