When Demon Queen Echidna launches her invasion of the human realm, the populace looks to the hero Leo Demonheart for protection. Blessed with insurmountable power, Leo effortlessly repels Echidna's army, thus saving the world. However, instead of gratitude or admiration, Leo encounters disdain and scorn from his fellow humans, who fear his overwhelming strength and believe it may eventually lead to humanity's demise. Exiled from the kingdom he once called home, Leo wanders aimlessly across the land until, one day, he hears rumors of Echidna's efforts to rebuild her army. Hoping that demonkind might accept him as an ally, he returns to the demon queen's castle and offers Echidna his help. Naturally, Echidna immediately rejects him. Still, not all hope is lost, as Leo manages to convince Echidna's four generals to allow him to work secretly under their supervision. Assuming the identity of a masked dark knight named Onyx, Leo solves the demon army's problems one by one—gradually improving the quality of life around the castle. Above all else, however, Leo wants to accomplish one goal: to learn why Echidna started the war in the first place.