Mitsuki-chan is now at an age where she is more interested in sex than in studying, and her brother, a ronin student, stays at home all the time and peeks at her when she masturbates or when she brings her boyfriend over to have sex with him. As usual, her brother sneaks into Mitsuki-chan's room to try to have sex with her at night, but fails. From that day on, the balance of power is reversed, and the brother becomes Mitsuki-chan's servant, and he falls to the point where he is treated like a farm animal by his sister...