The story sets the player as Hakurei Reimu into a situation where Gensoukyou has been divided up into various warring states. The ever mischievous Yakumo Yukari, the culprit behind the start of this war game, has sent out letters to all of the major factions of the world laying out the rules of the game and promising to grant one wish for the winning team.

[Edited From Touhou Wiki]