Touhou 7.5: Suimusou - Immaterial and Missing Power

There's a feast in 3 days, but while preparing for it, the girls are distracted by a strange mist that seems to have been happening around this same time for several years now. It isn't exactly a problem by itself, but since nobody knows what it is or where it came from, everyone tends to be suspicious of each other during the feast, which hasn't made it a very fun event these past few years. This year, the girls are determined to figure out the cause of this weird mist so that everyone can have a good time at the feast.