Caster has summoned a gigantic Horror. If it is not eliminated before it finds a new source of mana, the Grail War might have to be called off, to say nothing of Fuyuki and the entire world being put in danger. Rider appeals to Saber and Lancer, and the three Heroic Spirits decide to fight side by side. However, confronted by an unkillable monster with infinite regenerative abilties, Saber and the others have a difficult fight ahead of them.