In 1598, there is a county magistrate named Kim Dam Ryeong, in She County, Gangwon-do. He stayed at Yang's house by the seaside, and Yang took him to watch the mermaid he caught. Dam Ryeong asked Yang what he was going to do to catch the mermaid. Yang said that the fish oil extracted from the mermaid was of good quality and would not spoil even after being left for many years, and even the whale oil is hardly comparable to this grease, because of the nature of being rancid after a long time. Dam Ryeong pitied for this mermaid and took it back from Yang, and released it into the sea. In 2016, just as human beings would encounter traffic accidents even if they are unwilling, the mermaid comes to the land after the accident, and meets the first human being, Heo Joon Jae. After returning to the sea, the mermaid misses that man deeply. In order to keep the promise to meet the man again, she returns to land again and comes to Seoul. In the world of mermaids, she is the most arrogant and brilliant and clever, but in the human world, in the eyes of Joon Jae, she is just an idiot who eats a lot. This ignorant woman knows almost nothing about the world, and she believes what Joon Jae said firmly, and even learns the world through him. Unconsciously, he begins to feel guilty of lying to this woman. These two people who couldn’t have coexisted, start a love story full of pathos.