Lotte is a Mermaid Princess, which has reached the age and is allowed to explore the ocean alone now.

By swimming all alone, she saw a young man drowning by falling from a ship. After saving him, a demon called J would fulfill her wish to become a human under some circumstances. Lotte, who deeply wants to be by the side of this man, accepts J's conditions and become a human. However, the conditions will bound these two's life together...