A father-in-law discovers his daughter-in-law is cheating on his son.

He decides to confront and to discipline her into becoming a good wife.Of course, his disciplining is sexual - spare the rod and spoil the child, but he wants to use his own rod.
The reluctant DIL is overcome forcibly and aroused unwillingly.He keeps her on edge while forcing her to acknowledge her cheating.
More embarrassing for her is when he rims her, and makes her rim him.

This is followed by the obligatory blowjob.
Followed by the obligatory cream pie - amazing how they manage those with a condom on ;)
The FIL needs to find excuses to discipline her. The next time is over buying junk food for him. After that, the excuse is weak tea, and so on.
Ultimately, the DIL, of course, comes round to her FIL's thinking and becomes an obedient wife - to him. The husband, apart from two very short goodbyes, is nowhere to be seen, nor is the lover.

What makes this movie stand out for me is the acting by Rio Kuriyama. She does a great job of showing her resistance to FIL and even when he forces orgasms out of her, she is still reluctant. Only at the end is the reluctance turned to acceptance.

It also helps, for me, that Rio is very pleasing to the eyes. She is young - not yet 27 - and has a fit body and a pretty face.
Overall, I'd watch this and her again and again.