Eleven-year-old Grace Minnaugh is not a fan of big changes. She's miserable during her first weeks in the seaside town of La Toya, sulking like a spoiled brat. Her family embraces the California lifestyle and Grace decides to embrace the seductive beauty of the sea. One fateful morning, while taking a predawn swim, she is caught in a thunderstorm. Without the ocean skills to survive, she's sucked below the surface, convinced she will drown. Instead, a new life begins. Gills rip open from behind her ears and her legs fuse together to form a fishtail. Grace Minnaugh is a mermaid, and a gorgeous one at that. On land, Grace is still the same walking, talking social misfit she's always been. But in the salty sea, she's an underwater marvel. Grace decides not to tell a soul about her flip-floppy double life, but things get complicated when Grace befriends Alfie DeCosta, a kid who's obsessed with finding an elusive shipwreck off the coast of La Toya. Grace knows exactly where the shipwreck is. But she can't tell Alfie about it, or can she?