Yuuri Shibuya was leading a fairly ordinary life. That changed the day he was shoved into a toilet while trying to rescue a classmate from a group of bullies. Rather than just getting soaked, he is pulled inside. The next thing he knows, he finds himself in a world that vaguely resembles medieval Europe. If that isn't strange enough, he is told he is to become the next Maou (Demon King), simply because he has black hair and black eyes. The Maou is the King of the Mazoku (Demon tribe), who are coexisting not-so-peacefully with humans in this world. Much to his subjects' dismay, he is completely different from the rulers they are accustomed to. He is kind, considerate, a believer in justice, and unwilling to use violence to resolve conflicts. He is not exactly the type they want leading a country on the very brink of war. Now, Yuuri has to deal with trying to become a good Maou, while at the same time attempting to adapt to this land's customs and culture, all in a world where the tension between humans and Mazoku is reaching its peak.