[6] 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.

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.

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.

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.

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.

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).

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).

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.

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.

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.

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.

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.

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.

The show started with a strong 16.4% viewership rating nationwide, reaching as high as 20.7% for certain scenes, according to Nielsen Korea and surpassed the 20% mark in the metropolitan area by its sixth episode.

[49][50] The series also fared well overseas, having been exported to all major regions around the world, including Southeast Asia, the Americas and Europe.