After millennia in hell fighting for his survival, Belial claims the throne as the demon king. After his ascension to the throne, he promises the other demons that he will conquer the mortal realm, a place filled with greenery, food and resources so that his people don't have to fight for survival anymore. When Belial attempts to ascend to the mortal realm to prepare the gates for the demon army to invade Earth, an error in the guideline causes him to be confused with a summon of a human mage called Leila. Belial now soul-bound to Leila as her summon, tries to find a way to open the gates to the demon army to invade Earth while simultaneously helping Leila on her quest to revive her fallen family.