Synopsis
The series centres on the love story of Shim Cheong (Jun Ji-hyun), a mermaid, and Heo Joon-jae (Lee Min-ho), a witty con-artist. Focusing on rebirth, fate, and unrequited love, their tale is juxtaposed with the parallel story of their Joseon-era incarnations, the mermaid, Se-hwa, and town head, Kim Dam-ryeong.
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 live upon the land with Dam-ryeong, but she cannot turn her fishtail into a pair of human legs yet due to having not reached adulthood. Devastated by the news of Dam-ryeong's marriage, she decides to leave him forever. Nevertheless, Dam-ryeong's love for Se-hwa forces him to leave his wife on their wedding night and jump into the ocean to look for her. A 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, their paths cross again when they are adults as Se-hwa is found by town folks trapped in a cave after a storm, and brought before Dam-ryeong, now a widowed town official. While Dam-ryeong does not remember Se-hwa, he takes pity on her and sets her free. This angers the town inn head, nobleman Yang Seung-gil, who had plans of exploiting Se-hwa for his monetary desires. Yang plots to overthrow Dam-ryeong, with the help of his materialistic concubine, Hong Nan, as he 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 rescues Se-hwa, tortured by Yang and his concubine to extract her tears, which turn into rare sea pearls. Yang and his concubine are arrested, while Dam-ryeong returns Se-hwa to the sea so that she can recuperate.
Dam-ryeong, who has been experiencing dreams of himself, Se-hwa, and their foes in the future, fear that the same ill-fate that afflicts himself and Se-hwa will replay itself. To warn his future self, he commissions 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, mainly 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 due to the rumours. While Dam-ryeong is being escorted on a boat for his exile, Yang and his associates renew their search for Se-hwa by setting a trap using lanterns, and she is lured up to the surface, thinking it is Dam-ryeong's signal. Se-hwa comes under attack by nets, arrows, and harpoons from Yang's men. Urged by his suspicions after noticing the floating lanterns from afar, Dam-ryeong requests for the boat to turn around, and a fight ensues between the guards and Yang's men. 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 arms, 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 Se-hwa's hand as she dies and is lost to the ocean.
Yang and his concubine are executed for their crimes after the surviving officials clear Dam-ryeong of the false accusations. It turns out that Yang had extorted and poisoned a merchant; the merchant's son Park Moo avenges his father.
Joon-jae and Shim Cheong
In the modern-day, young Heo Joon-jae runs away from home after his parents' divorce and his father obtaining the rights to raise him. His father, Heo Il-joong (Choi Jung-woo), adopts stepson Chi-hyun (Lee Ji-hoon) after his marriage with Kang Seo-hee (Hwang Shin-hye). Joon-jae's mother, Mo Yoo-ran (Na Young-hee), after the divorce, works as a housekeeper after running out of money following the divorce. During his search for his mother, Joon-jae becomes a con-artist, using his good looks, wits, and skills in hypnotism in conducting scams with his mentor, Jo Nam-doo (Lee Hee-joon), and computer hacking genius, Tae-oh (Shin Won-ho). His affections are chased after by his college classmate, Cha Shi-ah (Shin Hye-sun).
Using the money earned from his scams, Joon-jae flies to Spain for a vacation. There he meets a mysterious woman with an unusual demeanour who breaks into his hotel room. He calls the police, who take her as a suspect for recent thefts in the local area, but Joon-jae retrieves her after being curious about the jadeite bracelet he saw on the woman's arm. Nam-doo receives a photo of the woman with the jadeite from Joon-jae and confirms over the phone that the jadeite is genuine and of unimaginably high price due to its quality. Meanwhile, Joon-jae is chased by gunmen hired by a rich businesswoman who had fallen for Joon-jae's scams. He brings the mysterious woman with him throughout the pursuit until they end up cornered on a cliff facing the sea. They jump into the water, and the woman revealed to be a mermaid unbeknownst to Joon-jae, kisses him to erase his memories of her but promises to follow him to Seoul.
Joon-jae returns to Seoul and is tormented by the gaps in his memories and Jo Nam-doo's claims of him being with a girl with expensive jadeite. After three months of travelling, the mermaid also arrives in Seoul, wandering around the city for days until she runs into Joon-jae. He recognises her to be the woman Nam-doo refers to from the pictures he sent to Nam-doo and interrogates her on her identity and why he cannot remember her, although she hesitates to answer. He brings her to his home and names her Shim Cheong. He allows her to stay at his home for several days to recollect and sort out his memories of her but makes her leave as she is reluctant to help him remember anything significant. That evening, she accidentally gets hit by Chi-hyun's car, after which Joon-jae decides to take her back to his house. After this, they steadily grow close to each other.
Linked by dreams, mysterious artefacts, including the jadeite bracelet, a vase with an image of a mermaid kissing a man in modern-day clothes, and the portrait of Kim Dam-ryeong with the inscription “everything is repeating itself", Joon-jae learns of Dam-ryeong's world and his fate, concluding that he and Shim Cheong are reincarnations of Kim Dam-ryeong and Se-hwa. Through his consultations with Professor of Neuropsychiatry Jin Kyung-won (Lee Ho-jae), he can retrieve his lost memories and the entire rendering of his dreams about Kim Dam-ryeong and Se-hwa. He can also hear Shim Cheong's inner voice; he learns that she is indeed a mermaid and that, unless she returns to the ocean, her heart will harden and eventually stop if he will not love her.
Kang Seo-hee, Joon-jae's step-mother, hires the fugitive, Ma Dae-young (Sung Dong-il), to kill Joon-jae as a part of her scheme to inherit Heo Il-joong's fortune. She also has secretly introduced Heo Il-joong with small yet potent doses of anticholinergic and monkshood extract, contributing to his failing health, particularly his vision. Chi-hyun dutifully nurses him, completely unaware of his mother's betrayal. Ma Dae-young attempts several times to murder both Joon-jae and Shim Cheong but fails. During an attempt to murder her, Ma Dae-young is surprised by Shim Cheong, after which she erases Ma Dae-young's memories and, in the process, she also learns the fate of their Joseon era incarnations Kim Dam-ryeong and Se-hwa.
Joon-jae and Shim Cheong live comfortably with each other despite conflicts and the fear of the impending ill-fate. Thanks to Shim Cheong, Joon-jae also reunites with his mother, Mo Yoo-ran, who quit her job as a housekeeper. On the other hand, Chi-hyun learns that his birth father is Ma Dae-young, Kang Seo-hee's longtime partner. He also learns of Kang Seo-hee's crimes and reluctantly consents to them. Kang Seo-hee finally kills Heo Il-joong, using a solid dose of monkshood as she becomes aware of Heo Il-joong's suspicions. Joon-jae, with the help of Shim Cheong, Jo Nam-doo, Tae-oh, and police officer-detective Hong Dong-pyo (Park Hae-soo), busts Kang Seo-hee, revealing her real name to be "Kang Ji-yeon" and her schemes. A maddened Chi-hyun grabs a police officer's gun as he's being arrested and shoots at Joon-jae, but Shim Cheong takes the shot to save him. Kang Seo-hee and Chi-hyun are both arrested, and while in custody, Chi-hyun commits suicide by monkshood poison. Shim Cheong, being a mermaid, survives the fatal injury and makes a miraculous recovery that astonishes hospital staff.
Joon-jae learns that Ma Dae-young is visiting Professor Jin Kyung-won. As Joon-jae, Jo Nam-doo, and Officer Hong Dong-pyo rush to get him, Ma Dae-young recovers his memories of his past life again and learns that the executions of his incarnation, Yang Seung-gil, and his concubine, Hong Nan, was carried out by the Park Moo, Jo Nam-doo's incarnation. Just as Professor Jin Kyung-won was about to be murdered, Joon-jae arrives with the police who arrest Ma Dae-young.
Shim Cheong is discharged from the hospital afterwards. However, she is still in recurring pain since she was shot at her heart. After noticing Cheong's discomfort several times in the absence of a complete recovery, Joon-jae decides to let go of Cheong so that she can go back into the ocean to regain her health. Before leaving, Shim Cheong erases Joon-jae's memories of her as the uncertainty of her return is enormous, even after he disagrees with the idea, and later, of everyone who knew her, except that of her friend, Yoo-na (Shin Min-ah), an elementary school student who is the reincarnation of a daughter of a mermaid and human fisherman.
Three years later, Joon-jae becomes a prosecutor, Jo Nam-doo works as a lecturer, and Tae-oh, now working as a white hacker, is engaged to Cha Shi-ah. Shim Cheong fully recovers and returns to Seoul to meet Joon-jae. Despite his lost memories, Joon-jae recognises Shim Cheong, and it is revealed that he recorded his life with Shim Cheong, in case his memories are lost again. They move to a house near the sea, and Shim Cheong becomes pregnant. Joon-jae ultimately evades the ill fate that had been in store for them with Kim Dam-ryeong's visions.
