Groper Train: Panty Hole (1990)

Takashi Sonoyama's wife Akiko is far from being a housewife, so he started cheating on her with Etsuko, whom he met on a train. He then tried to break up with Akiko and marry Etsuko, but... On the other hand, Wataru Todoroki's wife Kumiko, who works both jobs, comes home late every night, so he asks Iwabuchi, a private investigator, to investigate her background, but he says there's no sign of cheating. However, surprisingly...