Kagami Kenichi meets a demon murdering a woman while taking a shortcut home from school. He is rescued by Tsukishima Miyu, a gun-wielding girl. She explains to him that the demons were once humans and the only weapon against them is guns. He soon discovers his classmate, Hirai, is a demonized human when he is attacked by him. Later on, Miyu makes a contract with Kenichi and uses his soul energy to summon a "Demon Exorcising Weapon" and kill the demon...