Winter has come around in Gensokyo. One day, to a surprise, a geyser shoot up from the grounds near the Hakurei Shirine. Reimu Hakurei, the shrine maiden, would usually go and investigate on a such an unusual event. However, this time she was actually happy for such an event, as the heat from the geyser's heat would melt the snow, and form a hot spring that would attract visitors to the shrine. To crash her hope, as the denizens of the underground started making their way up to the grounds of Gensokyo. To human beings of Gensokyo did not see this as a problem.
Patchouli Knowledge, the magician of Scarlet Devil Manor, sensed danger in these youkais. As she fails to convince Reimu and the others about the youkais, she goes to visit the oldest being of Gensokyo, Yukari Yakumo.
As Yukari knows of what would happen, she promises Patchouli that she will send Reimu to investigate, as youkais of the above ground cannot enter the underworld.
Sorry if the description doesn't sound very well.. you can find the official prologue here: http://touhou.wikia.com/wiki/Subterranean_Animism:_Prologue
