The Holy Grail War has seven classes, yet the focus is always on Saber. Doesn't that get tiresome?
Perhaps the Holy Grail was tired of the same old formula too. Saki, a descendant of an obscure line of magi who knows almost nothing about magic, was practically forced into this Holy Grail War.
And the Servant whose fate became intertwined with his was neither a blonde swordswoman nor a white-haired archer who fought at close range.
It was that Assassin?!
Then the only way forward was to fight a Holy Grail War like an Assassin: anything but fair and square.
[An alternate timeline. The Three Great Families have been removed, the story is unrelated to the main plot, and one Servant is an original character.]