The world's gods are not myths or legends. They exist, unseen by humans, fighting their battles and destroying land and life. People regard the battles of such "Heretic Gods" as inexplicable natural disasters from which they cannot escape. Godou Kusanagi, a sixteen-year-old high school student, travels to Italy at the request of his grandfather to return a specific tablet to an acquaintance for safekeeping. Godou has no idea he'll be drawn into a battle between two Heretic Gods alongside Erica Blandelli, a self-proclaimed witch fighting to protect people. Fortunately, he defeats the god of war in mortal combat and becomes a "Campione"—or "God Slayer"—tasked with fighting Heretic Gods in order to save humanity.