The play is based on the legend of a mermaid in the earliest Korean folk story collection "Yuyuyetan", in which the magistrate of Seokgu County, Kim Dan-ryeong (Lee Min-ho), released a mermaid caught by fishermen back into the sea. It tells the story of a mermaid Sim Cheong (Jeon Ji-hyun) who came to the city hundreds of years later and was on the verge of extinction. She adapted to life on land and reunited with the genius con man Heo Joon-jae (Lee Min-ho) and fell in love with him.