Mio, who has been working as a model in Tokyo, returns to her parents' home for the first time in a long while. Her parents are happy to see each other again after such a long time, but her younger brother Jun has a cold attitude and seems to stay at home and not go to school. So she teases him by saying, "If you go to school, I'll let you rub my breasts..." but this awakens her brother's dormant sexual desire, and he roughly rubs her huge breasts and cums inside her in a childish sex act. From that day on, she is chased around the clock by her insatiable brother...