Hero Philina's party is on the verge of annihilation in front of the demon king's strength. Such the strongest demon lord proposes to exchange bodies with the hero for one year as a condition to avoid annihilation. Naturally, Ati, who is a priest and a childhood friend of Fillina, will stop her, but Fillina accepts this condition in order to protect her companions. Thus begins a new journey to subdue the demon king and the unique hero party of the naive demon king...!!