A Great Hero who was about to defeat the demon king dies when his clumsy ally mage accidentally casts a large area spell out of nervousness which in turn, killed both the Demon Lord and the Hero.

However, just when he thought he had died, he finds himself residing in a girl's body, and as if things couldn't get any worse, the original owner still resides in the body and holds dominant control over it. Now, there are two souls inside the body.

The Hero finds out that the girl is a demon and is one of the 20,000 candidates vying for the title of "Demon Lord" in the Demon Academy, he then decides to teach the girl to help her on her path. A Demon lord candidate with the qualities of a Hero, nothing could go wrong from this combination... right?