Merlin (voiced by Kelly Sheridan) is a girl who loves the sea. During a surfing competition, she encounters a dolphin named Zuma (voiced by Tabitha St. Germain), who tells her that she possesses the unique ability to breathe underwater because her mother is Calissa (voiced by Nicole Oliver), the Queen of the Sea—meaning Merlin, who had been kept in the dark all along, is actually a mermaid princess. 　　 Overwhelmed with surprise and joy, Merlin shares the whole story with her friends Fallon (voiced by Nakia Burrise) and Heidi (voiced by Maryke Hendrikse), both of whom are thrilled for her. Unfortunately, Merlin learns that something sinister and terrifying is happening in the underwater kingdom of Aquamarine, and she is the only one who can save the kingdom from peril. Thus, Merlin embarks on a wondrous undersea adventure. Amidst countless dangers, will Merlin be able to successfully complete her mission?