The Mukai siblings lost their parents when they were young. Her older sister Ai continued her efforts and she graduated from junior college and she became an office worker. On the other hand, her younger brother Taichi was playful and didn't study well, but Ai was very fond of her only family, Taichi. One day, Ai receives a phone call from a man named Nakata. "Your younger brother made a mistake on our group's sister." Ai, who was summoned to make up for her loss, was trampled on by Nakata and others in front of her brother, and was forced to have sex with her brother...
