Prologue: Sieg

In a desolate future.. 3000 years from our present.. a man named Sieg summons A Saber Class Servant . His intent.. to travel to the past.. to Camelot, to obtain the Holy Grail before it became corrupt so that he may obtain a precious wish for himself.

But why?

Meanwhile, in the Present day.. Chaldea, an angency that protects the Future of humanity faces a large problem. By next month, Humanity is destined to disappear from existence!
Can Mikoto, Saber, and their friends find the cause in Camelot? Or is humanity doomed to expire by next month?