When Phoebe flew to visit her American cousins in South Carolina, she thought just leaving her home was an adventure. Boating around the islands with the other girls led them to jokingly talk about finding a lost pirate's treasure, but when the girls stumble across a secret treasure cave they can hardly believe their fortune.
Under the influence of a full moon and an open treasure chest, the teenage girls all pull a bracelet and place them on their wrists. An accident closes the chest before they can loot more of the treasure and when the heavy stone door starts to trap them inside they flee with the hopes of returning later.
The next morning's swim brings Phoebe a new surprise when a magical glow surrounds her and moments later she finds her legs changed into a mermaids tail. Her cousin Emily's morning shower ends with a change into a mermaid as well and her friends quickly realize that they are all cursed.
While trying to discover a way to break the curse, the girls find that life goes on and they have to hide their new secret from friends and family alike in the fear of being discovered.
As new powers arise, the girls find that there are perks to being mermaids as well and soon some wonder if they even want to be cured despite the fear of people finding out their secret.
To further complicate things, a local boy, named Steve, becomes interested in Phoebe and invites the girls to a party. With a pool in the backyard and water balloons in the air, the mermaids quickly find new challenges to hiding their secret. Even a rain shower threatens to betray the girls and threatens to have them thrown into a laboratory, their greatest fear.
Will they be found out before the girls can remove the cursed bracelets?