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 Shim Chung (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.