Once upon a time, there was a beautiful young mermaid, named Arielle. She lived under the sea with her family and the other merpeople. She was always longing for something more. She kept a cavern that she filled with things - lost treasures from the world above the sea. She collected things for many years, and one day, started being brace enough to swim to the shore and fell in love with a handsome Prince. We all know the story, but not quite the end.
Arielle found her love, with the assistance from the evil sea hag. We know how the story went there. What isn't in the movie or the storytale that ended quite differently, is that after a time, she realized she did not age, while her love did. On top of that, her wish granted by the MerKing to be allowed to be human and marry her Prince was more than she thought. She could shapechange back to a mermaid whenever she chose to. She lived a happy life until her Prince died, and she returned to the sea, back to her family.
Nothing was quite the same, and she left her home and started wandering the oceans. She made a new dolphin friend and they ended up off the coast of California. She found a new cavern, and started filling it with treasures she found again. She was rediscovering herself, and slowly healing from the loss of her mortal homid husband. On one such trip, she ended up saving a fisherman, named Gerard. He became her friend, and she revealed her true self to him. Since then, he has helped her from time to time. Mainly in getting to know the present day society, and setting her up with a place to live on land, as well as selling off some of the treasures she finds under the sea.