When Molly Holmes moves with her family to Horseshoe Bay, her grandmother gives her an enchanted shell necklace that magically transforms her life. The power of the necklace transports Molly to the Undersea Kingdom, where she discovers that she is a Secret Mermaid and meets lots of new mermaid friends. But the evil Dark Queen Carlotta seeks to steal control from the good Merqueen Luna, by seizing the magic pieces of conch shell that help protect the Undersea Kingdom.