Little Scallop, princess of Mollusk Island, is gliding through the enchanted waters of Mermaid Lagoon with her twin mermaid friends, Surf and Aqua. But the moonlight magic shatters when Surf is captured and taken prisoner aboard the Carnivale - Ebenezer Crookshank's sinister floating circus. As Captain Hook and his band of cut-throat pirates close in on the ship and its booty, Can Little Scallop and her underwater friends find a way to save the missing mermaid?