Bao Rou Rou is a mermaid who sets out on land to find her relatives. She is tricked by the divine detective Gu Xing He into becoming his crime-solving partner. A madman who investigates a case meets a mermaid girl, and the two work together to solve a series of mysterious cases and the shocking conspiracy behind them. However, due to being framed by others, a long-lasting relationship turns into a tragedy of love separated by yin and yang.