Individuals who experience severe depression and anxiety can transform into dark demons when they lose control. Fortunately, there are individuals dedicated to eradicating these demons. Kagami Kenichi is ambushed by a demon while taking a quick route home from school. However, a gun-wielding girl named Miyu intervenes and repels the demon. Kagami's mundane existence comes to an abrupt halt as he pledges allegiance to Miyu through a kiss, resulting in his soul serving as her energy source and in the augmentation of her firearm. Kagami is promptly confronted with the grim reality that people can, in fact, metamorphose into demons.
