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.

But when she meets Finn, the new kid who seems to be awkward around others, but strangly  warms up to her. She wonders why her, why not anyone else in the resort. Is there something he is hiding. Something he wants to show her. When he begins to ask questions about her aunt, Kathy answers, thinking he is just a tourist and wants to know about the resort. Kathy, could never be more wrong.