Kono Naka ni Hitori, Imouto ga Iru! Mikadono Shougo is the son of an industrial magnate named Mikadono Kumagorou, who passed away. As a final wish, Kumagorou gave Shougo a condition before Shougo can inherit the family legacy — find a female mate at school. Shougo transfers into a school to fulfill that condition. However, he learns that he has a younger sister who was separated at birth. He does not know what his long-lost sister looks like, but she happens to be one of the students at his new school.