One May afternoon Yuri Shibuya was on his way home from school when he came across Ken Murata being harassed by bullies. When Yuri goes up to stop them Murata runs away and Yuri becomes their new target. They take him into the girls bathroom and shove his face in the toilet, but instead of just getting soaked Yuri is suddenly sucked into the toilet through a black hole. When he wakes up he's in a strange new place where no one speaks Japanese. Yuri comes to find out that he has been transported to another world and that he is actually a demon (魔族, Mazoku) and that he is the demon king (魔王, Maoh, Mazoku King) in this world. He is the king of a country known as The Great Demon Kingdom (眞魔国, Shin Makoku, True Mazoku Land).

He is taken to the Capital of his new nation by Günter and Conrad. When he arrives at the castle he meets Wolfram and Gwendal who find it hard to believe that Yuri is their new king. At dinner the next day they are talking and Wolfram insults Yuri's mother for being human and so Yuri gets upset and slaps him. The only problem being that in this other world a slap on the cheek is a marriage proposal among demon nobles. Wolfram is insulted by this sudden proposal and immediately challenges him to a duel by throwing his knife on the floor but one again Yuri, being unfamiliar with the strange customs in this world picks up the knife without knowing he was accepting the duel. The next day when they fight Yuri wins the duel by using magic that even he himself did not know he had, and so everyone accepts him as the true demon king.