In modern Seoul, we meet Heo Joon-jae (Lee Min-ho), a charming but tragic genius con artist. He poses as a prosecutor to swindle criminals, living a lonely life driven by the search for his estranged mother. Simultaneously, a mermaid from the deep sea—amnesiac about her past life as Se-hwa—gets caught in a fisherman’s net. After escaping, she tracks a jade bracelet that Joon-jae stole, leading her to land.