During a peaceful summer in Gensokyo, an unnatural scarlet mist appears without warning and covers much of the land. It is strong enough to block out the sun, causing the affected areas to become dark and cold. Reimu Hakurei, a shrine maiden working at the Hakurei Shrine, and Marisa Kirisame, a magician, set out to find the source of the mist. Their search will lead them to the Scarlet Devil Mansion and to its eccentric owners...