Hiroyuki Kagurazaka, a severely overworked corporate worker dies in his sleep at work without realizing. He reincarnates to another world as the 15 year old Celestina.

Celestina only remembers the memories and knowledge of her past life and nothing else. Slowly, she learns about her new life and the world she now lives in. A world of swords and magic. Eventually her curiosity leads her to wanting to explore this new world, to go on an adventure to see all the hidden wonders and treasures.

But there's a problem. Celestina isn't your average 15 year old girl. She is the only daughter of the infamous Demon King and the revered Holy Saint. A princess who is to either become the next Demon Queen or Holy Saint.