"It's about time I change bodies." A magician who lived for hundreds of years by taking the body of another was taking a stroll through the forest in search of a new vessel when he found a young boy who was on the verge of dying with a great number of slaughtered monsters before him. With the condition that the magician would protect Erena, the boy's companion, he made a deal that they would coexist in his body. However, what the magician did not know was that the boy was the swordsman that had been expelled from the hero's party.