The older sister, an elite career woman is actually a serious brocon. When she goes to clean her younger brother's apartment, who's in university and started living alone, she finds an anal porn video.
After watching the DVD, the older sister becomes interested in anal sex and begins to anally masturbate right then. When her younger brother Takashi comes home then, he sees a bizarre scene, but his crotch swells at the sight of his older sister's ass sticking out, so he forgets himself and plunges into her anus with great force. The older sister becomes Takashi's meat toilet for the sole purpose of pleasure.