Yuzuru, who entered a prestigious boarding school famous for table tennis and spent his days devoted to club activities, which made him resistant to women, has returned to his parents' home for the first time in a long time. Meanwhile, Yuzuru's older sister Ai has had a fight with her husband and has returned to her parents' home. Ai, who was feeling stressed, finds out that Yuzuru is still a virgin during a conversation and jokingly seduces him, but her naive adolescent brother takes it seriously and starts sucking on his sister's huge breasts... and ends up cumming inside her...