Одна из них — моя младшая сестра!

At his father's will, Shougo was transferred to an academy where a lot of girls attend. "While attending an academy find a girl that will be your partner" - in other words his father's will was for Shougo to make a girlfriend. On Shougo's birthday he receives a cake from an unknown person. Then he gets a phone call saying "Happy birthday, Onii-sama! I adore you very much." It looks as though his little sister, who was separated at birth also attends this academy, and is trying to get closer to him without letting him know who she is. Can Shougo "properly" reunite with his little sister, and make a girlfriend!?