Ryo Inasa journeys to her mother's island home after her mother's sudden death, determined to unravel the mysteries behind her mother's unexplained eccentric behavior and sexual conduct. Defying her mother's warning to avoid the island and any contact with a relative named Hotaruko, Ryo arrives and becomes ensnared in a dark island cult. This group seduces women into ecstatic trances before ritually sacrificing them to their deity, Master Mizuno.

Ryo forms a close bond with Ruka, her personal servant and lover. Despite Ruka's protective efforts and their mutual affection, Ryo fixates on Hotaruko. When Bungo gravely wounds Hotaruko, Ryo chooses to stay with her over escaping with Ruka. Subsequent revelations expose Hotaruko as Ryo's mother—not her cousin—and Itsuki as her grandmother. This makes Mizuno Ryo's uncle, though the high priest's dialogue implies Mizuno may also be her father. Bungo is revealed as Ryo's great uncle.

Under the influence of the island's red flowers and the cult's psychological manipulation, Ryo enters an altered mental state and consents to sexual activity with Mizuno, who inhabits a puppet form. Ruka intervenes by killing Mizuno, prompting Ryo to immediately kill Ruka in an abrupt, insanity-driven reaction. This act cements Ryo's tragic descent into the island's cycle of violence and corruption.

The conclusion reveals Ryo has a child, implying Mizuno's rebirth and the perpetuation of the sacrificial cycle. Ryo has green eyes, brown hair reaching her shoulders or chest, and a teenage appearance.