Katharine has never believed in the existence of mermaids. She thinks they are just stories to spite children into believing. Her Aunt Claire, the owner of the Mermaid Lagoon Resort, tells Kathy that  her attepts at keeping herself from the mermaiding buisness, are no longer. For the entire summer, Kathy must masgarade as a mermaid, or she can never live with her Aunt Claire again.