This story is centered around the man named Yi JiHyuk, who has spent 1000 years in a Medieval Western fantasylike world before returning back to the modernday Earth. When he returns, he finds that his home is facing monster threats, and that humans with superpowers have appeared to protect the innocent people of Earth.
The thing is, Yi JiHyuk himself lived like a true, bonafide Demon King in that fantasy world, and the only reason(s) why he returned home is to find some peace and easy going life. So, what will a man like that do?