After a certain event Yuusuke witnessed as a kid, he now by trying to make his step-mother to fully surrender her body to him.
Gibo - Stepmother's Sin is a very erotic hentai visual novel with incest themes and a complex psychological story.
You play as Yuusuke, a young man who once witnessed his mother's passionate adultery with another man, which crushed his ideal imagery of a happy family and make him believe all mothers are whores. After several years his parents divorced, his father married a young and beautiful woman Misako.
The moment Yuusuke saw her, his childhood traumas made him realise he can't accept her as the new mother. He saw what a mother can do and he needs to sexually punish Misako for all the mothers' sins.
Drawn in an 80s' fashion, Gibo - Stepmother's Sin is a visual novel that features a lot of different sexual themes, ranging from oral sex, anal and titfuck to themes like BDSM and urination.