母さんのオトコ ～野獣のような男に牝の悦びを覚えさせられる最愛の母～

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.