Inspired by a classic Joseon legend from Korea's first collection of unofficial historical tales, about a fisherman who capture and kidnapped a mermaid, this drama tells the love story between the son of a Joseon noble family named Kim Dam-ryeong (Lee Min-ho) and a mermaid he named Se-hwa (Jun Ji-hyun).When Dam-ryeong was a kid, he almost drowned but was saved by a mermaid. Following the incident, they became friends and Dam-ryeong named her Se-hwa, after his younger sister. They ended up falling in love, but one day Dam-ryeong was forced by his family to marry another girl. Devastated by the news, Se-hwa decides to leave Dam-ryeong forever and erased all his memories of her and their love. However, fate brings them back together and they tried to escape from humans who were going to separate them. Fearing that the same ill-fate that afflicts himself and Se-hwa will replay itself, Dam-ryeong sends an artifact to be discovered in the future to try and warn his future self. The fates of Dam-ryeong and Se-hwa become entwined with their modern day incarnations. Dam-ryeong was reborn as Heo Joon-jae, a charming con-artist. He meets a mermaid who looks like Se-hwa (whom he later named Shim Chung) in Spain, and she was quickly besotted by him. Linked by dreams and mysterious artefacts, Joon-jae learns of Dam-ryeong's world and his own fate.