A school where having sex replaces a normal grading system. The one at the top of the sex list gets the grades.

Everyone is blindsided by Yoshihiro’s milf sister Mao’s transformation. Suddenly donning a sadistic smile, she takes control of Yoshihiro, forcing him to remember what occurred when they were still kids. Shiori, in admiration of Mao, attempts to act similarly towards a now pissed off Yoshihiro, and is met with retalitaion from Yoshihiro instead, falling into his clutches…