In 1926, during the Japanese occupation of Joseon, 17-year-old Soo-a, an uneducated handmaiden serving a prominent Japanese collaborator, rescues the life of Eui-hyeon, an independence activist she finds injured on the beach. As she tends to him, Eui-hyeon shares the tale of the 'Little Mermaid' with Soo-a, a story of a sea princess who saves a prince from death. Enamored with Eui-hyeon, Soo-a is determined to see him once more, unaware of the tragic destiny that awaits the 'Little Mermaid' in her pursuit to reunite with her beloved.