If all these years you listen to stories of a mermaid saving a prince's life and she end up falling in love then, you tell the same exact storyline to the kids to make them stay quiet while babysitting them, you need to prepare your mind for a  different storyline and try to accept it even if you think that it's way better having the original storyline. 

This is a story of a mermaid falling in love with a human boy after being saved by him. The boy moved to Seoul and convinced himself that all his memories with a mermaid was just a dream he had as a child after reading fairytales or maybe it's because of the tiny accident he had before moving to Seoul.