Kaa-san no Otoko 2 ~Saiai no Haha ni Muragaru Osu~

The protagonist Yoichi Sakuragi lives alone with his mother, Shino, in a cheap flat.
Life without a father is not easy, but the two support each other and live a simple but happy life.

Yoichi loves his mother Shino, who smiles at him like that, as a son, but at the same time he is strongly aware of her as a member of the opposite sex.
Yoichi's best friend Ryo is also attracted to Shino as a woman, and while being careful not to let Yoichi know, he waits for an opportunity to tell her how much he likes her.
Then one day - unable to hold back his feelings any longer - Ryo tricks Yoichi and visits the apartment where the mother and son live while Yoichi is out.
When Ryo succeeds in getting a moment alone with Shino, he hugs her from behind and forcibly takes her lips.
Shino is surprised and tells him that she still loves her late husband and that she can't accept Ryo, her son's best friend. Ryo, thinking that Shino has rejected him, goes berserk and pushes Shino down on the spot, forcing her into a physical relationship with him. Shino hasn't felt desired as a woman in a long time. She feels guilty towards her husband and son, but she also finds pleasure in sex with the young and bold Ryo.

Afterwards, Ryo and Shino continue to seek out each other's bodies, at her part-time job, at the mother and son's flat, on dates, and at Ryou's house with his brother and father... Shino knows she shouldn't do this, but she gradually loses herself not only physically but also emotionally.