Can a woman do anything for the sake of love? Ha Ni is a mermaid princess in the underwater empire but she longs for the human world when she sees and falls in love with a man. She transforms into a human and follows Hyun Myung 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 remain a human?