Rosie is a little mermaid who loves dancing with dolphins and building towering sandcastles. But she's just moved to Trident City and doesn't have one single friend to join her. She's too young to go to Trident Academy with her older cousin Rocky, her parents are busy at work, and Manny Nanny, who takes care of Rosie, can be kind of boring, even though she's super nice. Then, Rosie meets Aqua and finds that having a new friend might be more trouble than it's worth!