Prequel to Fate/Prototype.

Young Ayaka Sajyou wakes up to a normal morning breakfast with her father and older sister, Manaka, unaware that a battle of frightening proportions has now begun in Tokyo. This battle is the Holy Grail War, the first of a series of mutual slaughters where seven Magi and seven Servants team up together to fight in a battle royale to win the Holy Grail, a mystical omnipotent wish-granting device to have their own wishes granted.

As each of the Seven Masters summon their Servants and fight, more of the mysteries surrounding the Holy Grail are revealed, culminating in a epic battle that will test loyalties, forge new alliances, and makes one question, who is the real Master in this Holy Grail War?