An epidemic of unknown origin strikes mankind, killing all adults and rendering the youngsters at the mercy of the vampire kind. Four years later, Yūichirō Hyakuya and his friends make a daring attempt to escape from the vampires, unaware that a mortal trap awaits them.

Yūichirō and his friends introduced to a new member, Mitsuba Sangū, and are then dispatched for his first mission as part of the Moon Demon Company, but when they spot a little girl in distress, Yūichirō ignores orders and rushes to rescue her, despite being warned that he is just being lured into a trap.