Adu is no ordinary human being. He is actually one of the 12 most powerful Demon Kings who, after making a pact with an angel, has been allowed to materialize into a human's body in the real world under one condition:

He must seek and destroy all of the other Demon Kings.

Being one of the most powerful Demon Kings, this sounds like an easy task, but the after-effect of becoming more human-like is becoming less demon-like. Becoming less powerful.

This is the story of Adu's journey as he embarks on a quest to rid the world of the Demon Kings and protect its people, all while hiding his dark secret from the very people themselves.