High school student Mikoshiba Ryouma was summoned to a different world for the sake of helping his summoner wage wars. Sensing the malice from the one who summoned him, he reacted by killing the summoner, one of the most important people in the empire. After the escape, he rescues a pair of twin sisters from the hands of bandits. The sisters who can use magic, swear to serve him as subordinates. Thus, Ryouma begins his journey on the path of the supreme ruler.