This series is divided into two parts: "When I First Met You" is the first part with 22 episodes (1-22) "Just Like an Old Friend Returns" is the second part with 20 episodes (22-42) This drama is a costume fantasy drama adapted from the novel "The Legend of the Mermaid", telling the story of the spirit master Ji Yunhe and the mermaid Changyi breaking through the shackles of their identities, going through numerous hardships and finally becoming a couple, and working together to protect the people. Ji Yunhe, the strongest spirit master in Wanhua Valley, met the beautiful and simple mermaid Changyi. The two met and fell in love with each other, and gradually became lovers from natural enemies, but obstructions from various forces made their relationship bumpy, and they even had to hurt each other and live in different places... Can they overcome all kinds of difficulties and stay together?