Naranda is a country where mermaids are said to have lived in the sea once upon a time. Yeonji joins the Naranda Historical Research Institute as an intern to prove it. She has been dreaming of a mermaid since she was a child. At the institute, she meets her team leader, Hae-rang, who looks like the mermaid from her dreams. She tries her best to impress Hae-rang, but she's not impressed with her... The mystery of her past life, sinking to the bottom of the sea with the kingdom of Naranda begins to unfold!