Living in the impoverished city of Arkham, Kurou Daijuuji works as a detective. His life takes an unexpected turn when Ruri Hado, from the Hado Financial Group, approaches him to search for a magical tome. Though reluctant at first, Ruri sways Kurou with a tempting offer of a large sum of money upon completion of the task. While searching for the book, Kurou stumbles upon Al, a beautiful girl with astonishing powers as a grimoire. Together, they enter a contract, granting Kurou the ability to wield extraordinary magic. As if fate has intervened, Al reveals to Kurou the existence of Demonbane, a powerful deus machina owned by the Hado Financial Group. Al quickly activates Demonbane to combat the mechanical threat of the Black Lodge, and thus begins the war between the Hado Financial Group and the Black Lodge.