Thora, a half-mermaid, must spend ten years at sea and ten years on land according to mermaid law. She starts off her life by sailing the seas with her mother and guardian, Mr. Walters, but after her 10th birthday she returns to the seaside town of Grimli where her father grew up. While in town, Thora deals with evil tycoon Frooty de Mare who wants to expand his Tooty Frooty Enterprises