Live free and be happy. ―These were the last words of Amemiya Yuya's sister, who was seriously ill. Despite her wishes, however, Yuya, who was suffering from the same disease as her sister, was supposed to end his short life without fulfilling his promise. For unknown reasons, Yuya was suddenly reborn in another world. Confused by his new life, Yuya begins his life as the second son of a count. One day, Yuya thinks about the possibility that his younger sister has been reincarnated in this other world just like him. In this world, there is a sister, a young fiancée, a slave, a maid, etc. - but one of them is his own sister