After helping his friend Ken Murata, Yuri Shibuya finds himself getting his head dunked into a toilet and being flushed down to another world. In this world people with black hair and black eyes are known as the double black and are feared. In this world their are two kinds of people. The humans and the Mazoku(demons(they look like humans)) who has hated each other since the war 20 years ago. Yuri is saved by a man named Conrad and taken away, just to be told he's the next Demon king!
Now Yuri has to deal with the war threatning to come, and a accidental marage proposal. With missions and events popping up, such as the 4 forbidden boxes and more, Yuri has to stand strong. Yuri is begining to learn how to be a King and prove himself to his people by making peace real. Not just a dream.
Luckly he'll have the help of his new friends, Conrad,Wolfram,Gwendal and Gunter. Being a king isnt easy, and Yuri has to protect his people, both on Eath and the strange land he must rule!
After waiting 3 whole days for Yuuri to wake up, everyone is getting worried. Finally Saralugei wants to help and use his sword to revive Yuuri, but no one really trust's him. The question is, can he really save Yuuri?!