Kairi Sisigou is summoned to the Clock Tower by Rocco Belfeban, who informs him that Darnic Prestone Yggdmillennia has gone rogue, claiming he possesses the Holy Grail that was stolen from Fuyuki during the Third Grail War and announcing his intentions to form a new Mage Association in direct conflict with the Clock Tower. Knowing that the Yggdmillennia group has already summoned Servants to fight for them, the Clock Tower has decided to activate an emergency system in the Greater Grail, triggering a Great Holy Grail War...