Osamu is the protagonist of the story, and he always has a sister complex. He has taken care of his little sister, Rio, from the moment she was born, as well as his other sister, Mio, since she joined his family from his father’s remarriage. Every day he was on cloud nine being together with his two younger sisters. Finally came the day when they started attending the nearby school that he also went to. When he saw them in their school uniforms for the first time, his heart started beating faster as he gazed upon their slightly matured figures. Could it be love? Yes, he had begun to have a (sexual) affection for them.