While in hiding at a remote convent, a king's daughter sees a magical being dragging a shipwrecked man to the shore. The creature is a mermaid princess - the youngest daughter of the Sea Queen - but she shares more with her human counterpart than her royal blood.

By saving a young man's life, both women have sacrificed their hearts. In one moment, the lives of the princesses, mortal and mermaid, are transformed forever.