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.

A maiden has come willingly to join Godou's ranks, but his current Knights have other plans.