Surplus Princess is a story about a mermaid who attempts to become a human being so that she can stay forever with the man she loves.
Ha Ni (Jo Bo Ah) is a mermaid princess in the underwater empire but she loves for the human world when she sees and falls in love with a man. She transforms into a human and follows Hyun Myung (Ohn Joo Wan) to live in a temporary house for people who are preparing for employment. The pragmatic Hyun Myung is trying to find a good-paying job to provide him with a comfortable life. But will he be distracted by Ha Ni, who has just 100 days to make Hyun Myung fall in love with her so that she can permanently remain as a human being?