In the American city of Snowfield, a new Holy Grail War is about to erupt. Such ruthless confrontations pit against each other magic experts known as magi, who ascend to the status of Master by summoning Servants—heroic spirits of the past. The winner is thought to have any one of their wishes granted.

However, it appears that the organization behind the present situation has merely managed to create an imperfect copy of the Third Holy Grail War. As the unusual ritual catches the attention of the Mage's Association in London, Flatt Escardos, the young apprentice of Lord El-Melloi II—a former participant of the Fourth Holy Grail War—travels to Snowfield and, to the great dismay of his teacher, joins the ranks of the Masters.

In this fake conflict where the line between Masters and Servants is blurred, the different factions will have a hard time preventing the city of Snowfield from plunging into total chaos.