The Corrupted Grail threatens to erase humanity itself which has been labeled as a plague after an unworthy Master had won the last conflict.
New Masters have been elected to stop the destruction, being aided by summoned Heroic Spirits of mankind, they'll have to unite forces and destroy the Corrupted Grail, but what then? Will the promise of a new vessel being generated after that, able to grant any wish to one Master, make them wait for the end of the current conflict until they become enemies, or will they try to eliminate the others before that, granting a safer path for their ambitions? Can they truly believe in what the Mages' Association told them, or is it all another manipulation attempt to make them do their dirty work?
The only thing they know for sure is, they'll have to fight if they want to see what the future holds for them.