Reimu got possessed by evil spirit, but she managed to keep control of her body for the time being.

This rare spirit is deterred by male genitalia, but the problem is it put a barrier all over her body except her armpits. The only way to get rid of the spirit is to make sweet love to her armpits.

But who she is gonna ask for such delicate favor? Rynnoske? That would be too embarrassing! She wouldn't be able to look him in the eyes again! But you are a random stranger, so it should not be a problem.

"Stranger, you are my only hope!" she pleads and rises her arms, revealing beautiful silky armpits. "Quickly, make love to my armpits! I don't know how long I will be able to keep the spirit from taking over my body!"