One is a cold-faced swordsman in the world of immortality, and the other is a poor history girl in modern society. Every month when the moon is full, souls exchange. In his world, she used science to subvert immortality, and in her world, he used immortality to defeat mortals. When the secrets of the two worlds were revealed layer by layer, they discovered that this exchange spanning thousands of years was not to become each other, but to reunite.
