It was said that in July 1999, the chains binding the despicable demon king in the depths of Hell will come loose and 666 demons will spill out into the human world, wrecking chaos upon mankind. However, when the date arrived and the gates of Hell opened, the demons quickly decided that it was impossible to rule over 7 billion humans when they themselves did not number more than 700, and chose instead to assimilate into human society. Ten years later, most people enjoy a peaceful life without knowing the events of the past. The demons settled in the small town of Misaki near the Kanto Sea and lived happily away from the public eye… until a girl who called herself the ‘demon king’ transferred into their school.