A husband takes his wife to a hot spring after realizing he has a cuckoldry fetish.
He agrees for a man named Endo to get alone with his wife, all so he can watch the proceedings.
Just the thought of them being all alone together gives him a stiffy.
By the end of the five days, his once apprehensive wife is moaning intensely from their copulation.
However, Endo makes a shocking proposition afterward...
"How about we have a contest? If I win, Rin divorces you, and becomes mine..."