For no apparent reason, large quantities of demons and youkai begin to travel from Makai, the world of demons, to Gensokyo, swamping the land and leaving Reimu Hakurei with a lot of extermination to do. Frustrated, Reimu tracks the source of the invasion to "The Cave in Which the Door to Makai is Said to Be," which lies in the mountains behind the Hakurei Shrine. Reimu decides that it is her duty as a shrine maiden to root out the cause of this mess. Unbeknownst to her, Marisa Kirisame has learned of her plans and decides to follow along with the hope of discovering some unique powers to steal within Makai. Elsewhere, Mima, the evil spirit of the Hakurei Shrine, and Yuuka Kazami, the youkai, decide to travel to Makai as well, although they have their own reasons for doing so.