Unexplored territory known as inscrutable and Hell's Gate (The Gate of Hell), Hei appeared suddenly in the past, Tokyo depriving their original sky. As a consequence, also appeared people with special abilities, the Dolls puppets and Contractors.
Puppets are emotionless mediums, while contracts have certain special powers, and appear to have as its main purpose is to kill. Intelligence agencies around the world use Contractors to conceal secrets relating to the Hell's Gate, and carried out violent battles for possession of information.