A horror RPG following NYPD rookie Aya Brea, Parasite Eve opens on the night of a Manhattan opera, where a singer named Melissa undergoes a horrifying transformation and ignites a fire that kills most of the audience. Aya, attending the performance, survives and discovers that Melissa's change is tied to a phenomenon involving the cell's mitochondria. As the mystery deepens, Aya is drawn into a struggle against Eve — a being born of Melissa's mitochondria and bent on wiping out humanity to birth a new dominant species. With the help of scientist Kunihiko Maeda and detective Daniel Dollis, and aided by the awakening powers of her own unique physiology, Aya battles Eve and her monstrous spawn across six days, building toward a final confrontation with the fate of the world in the balance.