As the demons' era dawns, the ailing Dr. Faust predicts a looming evil. Unable to combat this malevolence himself, Faust bestows the duty upon a young lad, Shingo Yamada, marked by destiny as the fated demon slayer. With a birthmark as proof, Shingo is chosen for the task. Calling forth a reluctant devil named Mephisto from the depths of hell, Faust orchestrates humanity's slim chance for survival. Bound by a blood pact to protect mankind, Shingo and the indifferent Mephisto embark on a perilous journey to wage war against the supernatural forces threatening the world.