From the introduction:
Gensokyo is divided!
The land has been split up into various factions, all fighting each other for supremacy and control of Gensokyo. Now girls from all Touhou, including the PC-98 era, lead armies to battle it out with each other. Yet at the same time, darker forces have made their move: Succubi from the Dream Dimension are flooding into Gensokyo, intent on turning this dimension into their lewd paradise. Tentacle monsters from beyond have burst out of the ground, overrunning many civilians and spreading from there. Outsiders have broken in from the Outside World, intent on performing depraved experiments with the girls. Hobgoblins run amok, violating anyone they please. Amidst the chaos, bandits have sprung up, looting and lewding whatever falls into their clutches. Vigilantes form their own self-defence committees to protect themselves against the warring factions (and get a piece of the action). Cultists of dark gods (of tentacle rape) emerge. You will be thrust into the middle of this, just as the factions are gearing up to battle to control Gensokyo once and for all!