A world exists where monsters called Utsuro feed on nectar, a form of bioenergy that only young girls possess. These creatures are relentless, and for any girl under the age of nineteen, the fear of being consumed is a daily reality. Iruka Natsumi knows this terror all too well. She is a member of Hyakkaen, an organization that fights the Utsuro, yet she lives in constant fear of becoming their next meal.

Natsumis life takes a dramatic turn with the sudden appearance of a mysterious girl named Sagara Asuka. Asuka wields overwhelming power against the Utsuro and makes a bold declaration: she will single-handedly destroy every last monster in the world. She then informs Natsumi that they will live together from now on. The reason is the nectar within Natsumi, which will only disappear once she turns nineteen. Until that day arrives and she is no longer a target, Asuka vows to protect her.

The setting is a contemporary world invaded by these invisible or monstrous threats, forcing the creation of specialized fighters to combat them. Asuka, however, is not just a typical warrior. While she demonstrates incredible strength in battle, she harbors a significant secret that drives her mission. The narrative follows this unlikely pair as they navigate their forced coexistence. The central conflict stems from the countdown to Natsumis nineteenth birthday, creating a powerful ticking clock. The primary narrative arc begins with their first encounter and Asukas integration into Natsumis life, establishing their tense partnership where a girl afraid of being eaten must trust a warrior with a hidden past to keep her alive until she is no longer prey. The story blends supernatural action with the intimate dynamics of a girl-meets-girl drama set against a desperate struggle for survival.