Utsuro is an unidentified monster whose favorite food is the "nectar" bioenergy that only young girls possess. Iruka Natsumi, a member of Hyakkaen, an organization that fights Utsuro day and night, lives in fear of being eaten by Utsuro. One day, a girl named Sagara Asuka appears before her. Asuka declares, "I will defeat all the Utsuro in this world with my own hands," and tells Natsumi that they will live together until she turns 19, when the nectar disappears from her body. Asuka shows overwhelming strength in her battle with Utsuro, but she has a secret... (Source: Natalie, translated)