Kusanagi Godou is entrusted with a stone grimiore by his grandfather to deliver to its former owner, an old friend, in Italy where he encounters supernatural happenings. Meeting the beautiful knight, Erica Blandelli, he is drawn into the battle between rogue gods, and uses the power of the grimiore to defeat the Persian god of victory, Verethragna, gaining his powers, becoming a god slayer, also known as a Campione.

Godou must battle Metis, the snake goddess of death and darkness. A being that's only goal is to bring destruction to the world.