Okaa-san to Koubi ~Tojikomerarete Futarikkiri Sekai ni Nattara Hoka ni Nani Suru?~

The gentle and beautiful mother, Chizuru, and her devoted son, the protagonist. But their happy family of two was shattered on the day a once-in-50-years typhoon struck.

Trapped inside their house by a landslide, the two joined forces, encouraging each other as they waited for rescue. But when the protagonist spies his mother masturbating in the bathroom, driven by animalistic desire, they engage in a forbidden relationship...

At first, both mother and son were tormented by guilt over their transgression, but gradually they could no longer resist the pleasure of their forbidden act, drowning in the illicit pleasure between mother and son.

“From morning till night, anywhere and everywhere, mother and son repeated their forbidden sex, greedily devouring each other's pleasure...”

[From official site]