As a child, Yuu Kusaka made a vow upon a shooting star that he would stand on the side of justice and defend the weak. However, the death of one of his friends robs him of the desire to live up to that vow, and he turns a blind eye to the misery of others, using the mysterious power he possesses as a means of making money instead of helping those in need. His days of living only for himself continue, until he's forced to fight for his life against another classmate with powers similar to his. He's saved by a mysterious girl named Eimu Yoake, who also has strange powers.
Eimu reveals that his powers stem from a creature known as an Ayakashi, a parasitic being that grants the user special abilities at the price of slowly draining the life of the host. She helps him fully awaken his own. Now aware of the ayakashi he possesses, Yuu must live up to his childhood vow, putting his ayakashi to use and fend off those who would use their own for evil, all while unraveling the mystery behind the death of his friend.