Transfer student Yuuki Yuriya does not participate in church services or Bible study, so he is rumored to be the culprit behind the recent "demonic ceremonies" occurring at Seirei Academy. Truthfully, he has never before been involved with the supernatural—until one evening, when he witnesses an unknown girl jump from the school rooftop. Next to her bloody corpse, he finds a mysterious book containing instructions he feels compelled to follow. Yuuki is shocked when he successfully summons Demon King Belial, who offers to enter into a contract with him.

God bestows miracles unfairly and according to his own will, whereas demons grant wishes to specific humans who are deserving. Yuuki is tempted by these words, but he is also skeptical, so he asks for the dead girl to be resurrected in demonstration of Belial's powers. In the next instant, he is transported to a desolate battleground, where he learns that God has abandoned him for the sin of seeking assistance from a demon. Now, he must fight against other contractors in this purgatory called Cartagra.

Regardless of the outcome, Yuuki's soul is forfeit to Belial. However, if he can survive, then he may realize his desire to purge all evil in the world and even prosecute the unjust God.