Legend of the Blue Sea^[2] (Korean: 푸른 바다의 전설; RR: Pureun bada-ui jeonseol) is a 2016-2017 South Korean television series starring Jun Ji-hyun and Lee Min-ho.^[3][4] 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 of a con-artist and a mermaid who travels across the ocean to find him.^[5] It aired on SBS every Wednesday and Thursday at 22:00 (KST) started from 16 November 2016 until 25 January 2017.

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.

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.

In modern day, young Heo Joon-jae runs away from home after the divorce of his parents 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 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 after the divorce. In 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.

Using the money earned from his scams, Joon-jae flies to Spain for a vacation. There he meets a mysterious beautiful woman 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 from Joon-jae a photo of the woman with the jadeite and confirms over the phone that the jadeite is genuine and of unimaginably high price due to its quality. Meanwhile, Joon-jae is being 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, who was 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 by Jo Nam-doo's claims of him being with a girl with an expensive jadeite. After months travelling, the mermaid also arrives in Seoul wandering around the city for days, until she runs into Joon-jae. He recognizes her to be the woman Nam-doo is referring 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 his memories of her but just after he makes her leave as he was unable to remember anything significant, she gets hit by Chi-Hyun's car that evening, due to which he takes her back to his house. After this, they steadily grow close to each other.

Kang Seo-hee hires the fugitive Ma Dae-young (Sung Dong-il) to kill Joon-jae as 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 treachery. Ma Dae-young attempts several times to murder both Joon-jae and Shim Cheong but fails. After trying to murder but caught by Shim Cheonh, she erases Ma Dae-young's memories and, in the process, learns the fate of their Joseon era incarnations Kim Dam-ryeong and Se-hwa, which Ma Dae-young recollected again over time.

Linked by dreams, mysterious artifacts, 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 also learns of Dam-ryeong's world and his own 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 is able to retrieve his lost memories and the full 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 sea, her heart will harden and eventually stop if he will not love her.

Joon-jae and Shim-cheong live comfortably with each other despite conflicts and the fear of the impending ill-fate. Joon-jae also finds his mother Mo Yoo-ran, who quit her job as housekeeper. On the other hand, Chi-hyun learns that his real father is Ma Dae-young and he is Kang Seo-hee's lover. He also learns of Kang Seo-hee's crimes and reluctantly consents to them. Kang Seo-hee finally kills Heo Il-joong, using a strong dose of monkshood. 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-hyun" and her schemes finally revealed. A maddened Chi-hyun grabs a police officer's gun as he's being arrested and aims at Joon-jae but Shim Cheong takes the shot to save him. Kang Seo-hee is detained while Chi-hyun later kills himself with monkshood poison.

As Shim Cheong, being a mermaid, survives the fatal injury and 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 and learns the death of his incarnation, Mr. Yang, and Yang's concubine, revealed to be by the hands of Park Moo, Jo Nam-doo's incarnation. Just as Professor Jin Kyung-won was about to be murdered, Joon-jae arrives, and Ma Dae-young is seized by the police. Shim Cheong is discharged afterwards, but does not recover completely as she was shot at her heart. In order to fully recover, she decides to go back into the water. Shim Cheong erases Joon-Jae's memories of her, even after he disagrees with the idea and later, of everyone who knew her, except that of her friend named Yoo-na (Shin Rin-ah), a young girl and returns to the ocean.

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 (Shin Hye-sun). Shim Cheong fully recovers her health and returns to Seoul and returns to Joon-jae. Despite his lost memories, Joon-jae recognizes 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 evaded the ill fate that had been in store for them with Kim Dam-ryeong's visions.

Filming began on August 19, 2016 in Goesan, North Chungcheong Province, South Korea. The production team then flew to Palau to film underwater scenes.^[22] The cast and crew flew to Spain on September 11 where they filmed in A Coruña, Ribadeo, Lugo, Tossa de Mar and Castell de Santa Florentina^[23][24][25] in Canet de Mar, as well as Begur (Es Cau Swimming Pool), Sitges (Town Hall and other backdrops) and Barcelona (exteriors of Palau de la Música Catalana, etc.),^[26][27] among other places in Catalonia and Galicia (Spain).^[28][29] The first script reading was held on October 8, 2016, two months after production began, due to the busy schedules of the actors. It lasted three hours and covered the first four episodes.^[30]

This drama served as a reunion for Lee Min-ho and Kim Sung-ryung, who had son and mother relationship in the 2013 SBS hit drama across Asia, The Heirs, for Lee Ji-hoon and for Shin Hye-sun who debuted together in the 2012 KBS2 hit school drama School 2013^[31] as well as Jun Ji-hyun, Hong Jin-kyung, and Na Young-hee who previously worked together in the 2013 SBS hit drama My Love from the Star.^[32]

Due to the star power of Lee and Jun, the series maintained the first-place ranking for its timeslot and surpassed 20% viewership ratings.^[47][48] The series also fared well overseas, having been exported to all major regions around the world, including Southeast Asia, the Americas and Europe.^[1] Despite the show's success, it was criticized for lack of originality, as many viewers felt that the 'parallel timeline' and 'romance between a human and supernatural being' were recycled elements of writer Park's previous series My Love From the Star (2013-2014)^[1] Additionally, it was slammed for bad editing, loose plot and lack of chemistry.^[49] After the release of director's cut for the first two episodes which included additional scenes, it was acclaimed to be good and many felt that these episodes would've done much better than the previous cuts.

However, Jun's performance as a mermaid was met with overall high praise; with The Straits Times saying that she is "brilliant as the otherworldly goofball who is blissfully unaware of the mission to save her self-worth" and Yonhap News Agency highlighting her "trademark lanky, slender yet at times powerful and dramatic performance".^[50][51]

In the table below, the blue numbers represent the lowest ratings and the red numbers represent the highest ratings.