In a time of strife where human-youkai interactions appeared to be at their worst, Yakumo Yukari stumbled upon a certain "something", that "something" being a lost child. Little did the Youkai of Boundaries know of what the future then held in store for her, of how this child would change everything.

In a series of events that include: being rudely awakened to an unpleasant surprise, right in the middle of the night; getting thrown into one of Yukari's gaps; and meeting a mysterious blonde girl in red... Hakurei Reimu has only lived through what is the opening scene - the opening scene of one of Gensokyo's greatest Incidents ever to come.