When Sophie Swankowski surfaces from the freezing waters, she finds herself in an ancient castle in Poland and in the center of an ages-old battle. Even with her magic powers, the strength and wisdom she learns from her companions in Warsaw, and the help of her gruff mermaid guardian, Syrena, how can one thirteen-year-old from scrappy Chelsea Massachusetts, really save the world?

Luckily, Sophie won't be alone. As she connects to other girls around the globe who have been training, just like her, for this very fight, she begins to think she just may become the hero she's meant to be. But when she has to face the pure source of evil alone, using all the strength she has to keep it from destroying everything, how easy it would be to simply give up and join the other side...