The 4th Holy Grail War, a war that will determine the direction of the future. At the start of the war, young Shirou summons a Caster on accident. Together they will determine the fate of the war. Who is this new Caster and what is her objective? Will she lead to a reversal of fate, or are the participants still doomed to tragic fates? And what are Rin, Sakura and Illya doing here?