The television series centers on the love story of Heo Joon-jae (Lee Min-ho), the son of a rich businessman who becomes a clever con-man after his parents’ divorce, and a mermaid named Shim Cheong (Jun Ji-hyun). Focusing on rebirth, fate, and unrequited love, their tale is juxtaposed with the parallel story of their Joseon era incarnations, town head Kim Dam-ryeong and the mermaid Se-hwa.
Dam-ryeong and Se-hwa
During the Joseon era, a mermaid saves the son of a nobleman, Kim Dam-ryeong, from drowning. Following the incident, the two children become close friends and Dam-ryeong names her Se-hwa, meaning ‘a bright, shining child’, after his deceased younger sister. Despite being from two different worlds, they fall in love until Dam-ryeong is forced to take up an arranged marriage. Se-hwa plans to turn her fishtail into a pair of human legs and live up on land with Dam-ryeong, but due to having not reached adulthood yet is unable to do so. Devastated by the news, she decides to leave him forever. Nevertheless, Dam-ryeong’s love for Se-hwa forces him to leave his wife after the marriage and jump into the ocean to look for her. The distraught Se-hwa rescues him from drowning, and promptly erases his memories of her and their love so that he may not look for her again.
After years of separation, the two grow as adults. Their paths cross again as Se-hwa is found by town folks trapped in a cave after a storm, and brought before Dam-ryeong, now a town official. While Dam-ryeong does not remember her, he takes pity on her to set her free, angering the town inn’s head and nobleman Yang, who had plans to steal Se-hwa for his own and overthrow Dam-ryeong. With the help of his materialistic concubine, Yang murders those in his way and spreads a rumour that Dam-ryeong is unfit to rule as he is bewitched by a wicked mermaid, allowing him to capture Se-hwa as the village falls into panic. Dam-ryeong arrives too late to save Se-hwa, who has been tortured by Yang and his concubine to extract her tears, which turn into rare sea pearls.
Dam-ryeong, who has been experiencing dreams of himself, Se-hwa, and their foes in the future, fears that the same ill-fate that afflicts himself and Se-hwa will replay itself. To warn his future self, he commissions a soon-to-be artifact—a time capsule containing a portrait of himself with a message to his future self that “everything is repeating itself” and he has to protect the reincarnation of Se-hwa from danger, particularly due to the reincarnations of Yang and anyone related to him.
A furious Dam-ryeong is determined to kill Yang and expose all his crimes, until he is exiled and removed from his position by the court. While escorted on a boat for his exile, Yang and his associates renew search for Se-hwa. Urged by his suspicions, Dam-ryeong requests the boat to turn around and a fight ensues with Yang and his gang, with Se-hwa being attacked under water. As all the men aim their harpoons, Dam-ryeong plunges into the water just as a harpoon, hurled by Yang’s son, impales him from behind, saving Se-hwa. As Dam-ryeong dies in her hands, Se-hwa thrusts the harpoon deeper through his body and stabs herself. The boats sink, carrying Dam-ryeong’s belongings with them, and Dam-ryeong’s jadeite bracelet, which he gifted Se-hwa earlier, slides off from the dying Se-hwa’s arm and is lost to the ocean. Yang and his concubines were executed by sword and monkshood poison, respectively, for their crimes after the officials who survived from the fight cleared Dam-ryeong of the false accusations.