Gensokyo has settled down after the Spring Snow Incident that occurred in Perfect Cherry Blossom. Reimu Hakurei and the others are currently having a feast every three days at the Hakurei shrine. During these feasts, a strange mist appears. The playable character decides to get the cause of the never-ending feast schedule and the mist. This incident was later called the "Night Parade of One Hundred Oni Every Four Days" by Aya Shameimaru in Bohemian Archive in Japanese Red. (Touhou wiki)