After the death of her father, the king of Hillvalley, Princess Stella is thrust into a bitter succession conflict. Her stepsister Illusia, driven by ambition for the throne, makes repeated attempts to assassinate Stella. Each attempt, however, is thwarted by inexplicable and gruesome events. The truth is far stranger than political rivalry: Stella is host to a parasitic entity named Coron, the reincarnation of a modern Japanese man, who has taken up residence inside her body. Coron controls a network of tendrils throughout her system, and his sole priority is her protection, turning every assassination plot into a bloody spectacle.

The setting is a fantasy kingdom with a medieval European aesthetic, where court intrigue and dark fantasy horror intermingle. As the story progresses, the conflict expands beyond Illusia's schemes. Stella is branded as possessed by a demon, prompting the arrival of an exorcist who is himself controlled by another parasite, leading to battles between these grotesque entities. Stella's betrothed, Prince Radwin of the Dragonia Empire, arrives and proves to be a volatile ally, while her stepbrother Saizer becomes an antagonist corrupted by a similar parasitic infection. The narrative moves to the imperial capital of Dragonia, a den of conspiracies, where Radwin faces a trial by combat and Stella must act as his champion in a duel, forcing her symbiotic relationship with Coron into increasingly public and dangerous confrontations.