It is the Fourth Holy Grail War and Kariya Matou prepares to summon his Servant. However, Zouken had failed to secure a relic of a certain Knight of the Round Table, thus requiring Kariya to do the summoning ritual without one and relying on the Grail to choose a Servant for him.
Within the Holy Grail, the spirit of Angra Mainyu manipulates the system and bring forth a Heroic Spirit not from this world. Specifically, a certain teenaged parahuman.