The Key (1974)

3.6 | May 04, 1974 (JP) | | 01:30

https://www.nikkatsu.com/movie/25291.html

A college professor wants to bring the joys of sex to his frigid wife. He convinces his son in law to sleep with her because the idea arouses him, while also making him insanely jealous. The professor keeps a journal of his fantasies in a locked cupboard and hopes that his wife will find the key and read it.