Shim Chung, a mermaid from the Joseon era, follows her true love to modern-day Seoul, where she meets his lookalike, Heo Joon Jae — a charming con artist drawn to her mysterious jade bracelet worth $6 million. With nowhere else to go, she moves in with Joon Jae and his partners, Jo Nam Doo and Tae Oh. As she adjusts to life in the human world, she finds herself competing for Joon Jae's heart with researcher Cha Shi Ah, all while a deadly killer sent by his stepmother closes in on them.