Inspired by a classic Joseon legend from Korea's first collection of unofficial historical tales, about a fisherman who captures and releases a mermaid, this drama tells the love story between the son of a Joseon noble family named Dam Ryeong (Lee Min-ho) and a mermaid named Se Hwa (Jun Ji Hyun).

Shim Chung finds herself transported to modern times. She is caught by Heo Joon Jae, a charming but cold con artist who is the doppelganger of Dam Ryeong. But in the present time, Joon Jae works with Jo Nam-doo (Lee Hee Joon), a skilled conman who guides Joon Jae to become a genius scammer. Coincidentally, Joon-Jae’s junior, Cha Shi-ah (Shin Hye-sun), who works as a researcher at KAIST, may be Shim Chung’s only hope for surviving in her strange new world.