Keita has three imoutos: Erina, Tomoka and Yuzu. They may seem to be normal siblings, but he actually lusts after them, sneaking into their bedrooms at night and masturbating to their cute sleeping faces. At the same time, the oldest imouto Erina had fallen in love with him since he had always been by her side since she was born. However, she felt that there’s no way she could admit to having these feelings and continued to hide her true feelings. One day, a girl named Saori joined their family via their father’s remarriage, becoming their older sister. She was beautiful, kind, smart, great at housework and cooking – perfect in every way. Saori quickly became close with his imoutos and Erina found someone that she could confide in. Following Saori’s plan, his imoutos begin to launch a full-on assault on him.