Now, the Servant Nero Claudius and her Master have won the Holy Grail War and have obtained the universal wishing mechanism, the Holy Grail.
They also obtained the Regalia, a ring that proves their kingship. With it, they rule over their once-enemy Servants and start a new beginning on the moon's new world.
But a new enemy has appeared before them. And this enemy has something that shouldn't exist: another Regalia.