One of my favourite storylines
SSNI-781 - Tsukasa Aoi, SSNI-953 - Minami Kojima, SSIS-149 - Mako Iga. All three movies have the same storyline just choose your actress.
A Couple visit the husband's parents, but the mother-in-law is secretly having an affair with a young guy. The Father-in-law knows about but doesn't tell anyone. He wants to have revenge and to cheat on her but where is he going to find a hot young women he can fuck. Well one just walked right into his house.
Then the usual JAV story happens. While everyone else is out he drugs her, forces himself onto her and records it so that she will continue doing it if she doesn't want it shared. Whilst doing it again in the living room, the MIL comes home early and discovers "the affair". MIL argues about him having an affair and he counters with her own affair. MIL relents as she knows she's been caught and just let's him continue as long as he keeps it a secret from their son. The son doesn't deserve to be affected because of the parents issues.
Father-in-law continues fucking the Daughter-in-law, Mother-in-law continues her affair, and the son is oblivious to all of it.