Kaa-san no Otoko ~Yajuu no You na Otoko ni Mesu no Yorokobi o Oboesaserareru Saiai no Haha~

The protagonist, Shin'ichi Kosaka, has a secret that he cannot tell anyone.
It is that he is in a sexual relationship with his mother, Mizuho.

While ostensibly acting as a son and as a husband behind closed doors, the two love each other and have been carrying on a forbidden relationship.
Their happy life together, which seemed like it would last forever, is suddenly destroyed by a man.

The man's name is Onisawa Shigehiko.
He was a violent man who had long been attracted to Mizuho's good looks and often visited the mother and child. Onisawa discovers the mother and son's secret and demands physical relations with Mizuho in exchange for not revealing their secret.
In order to protect her life with her beloved son, Mizuho ends up giving herself to Onisawa.

Eventually, her ripe body becomes accostumed to Onisawa's hands and she becomes a prisoner of pleasure.......