Utsuro are unidentified monsters that feed on nectar, a form of bioenergy possessed only by young girls. Iruka Natsumi lives in constant fear of being eaten by these creatures, even though she is a member of Hyakkaen, an organization that fights against them day and night. Her life changes dramatically with the sudden appearance of a powerful girl named Sagara Asuka, who declares her intention to destroy every Utsuro in the world with her own hands. Asuka then makes a surprising demand: she and Natsumi will live together until Natsumi turns nineteen, the age at which the nectar disappears from a girl's body, rendering her no longer a target. The story follows this unlikely pair as they confront the Utsuro threat, with Asuka displaying overwhelming strength in battle while harboring a significant secret that drives her mission. The setting incorporates a school environment as the backdrop for much of the action and character dynamics. The central conflict revolves around the race against time to survive until Natsumi comes of age, the mystery of Asuka hidden nature, and the ongoing war against the monsters that prey on young women.