What is the basic plot of Touhou 7.5: Immaterial and Missing Power?

You mean it's not clear from the translated summary? ;)

This game is fantastic fighting game. Girls do their best now and are preparing Please watch warmly until it is ready. Greatest mesmeric mist of all time. This magic mist is missing power!

Still, a slightly more coherent description is given here:

While making preparations for the feast three days away, Reimu is distracted by a strange powerful mist that is covering Gensokyo. She and many other inhabitants of Gensokyo (such as Alice and Patchouli) go to investigate.

Suika Ibuki is an oni. Although legends say all the oni are gone, Suika has decided to reunite the humans and oni by attracting them through partying. Using her special ability to spread across the land, she causes the humans and youkai to have feasts and parties. The more drunk she gets, the stronger she becomes, and she carries a gourd treasured by the oni, which never empties of sake.

Still pretty bizarre, but then, that's practically the definition of anime...

Note that there are further dialogue translations provided in that first link, that will probably prove helpful if you really want to try and understand the details as the story unfolds. Good luck with that. ;)