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.

After passing the final test, Yūichirō and his classmate Shihō Kimizuki obtain their own Demon Weapons, but when Yūichirō's friend Yoichi Saotome ends up possessed by his weapon, the duo must find a way to bring him back to his senses without killing him.

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.