While Luanne and Heatheria enjoy a day off from fighting Evil by fishing on La Orilla beach, Luanne accidentally hooks a swimmer with her fishing rod--only to discover that she has hooked their next adventure, as a mermaid ends up asking for their help in vanquishing Mantene, the Sea Witch of Oceanus. Marina tells Luanne and Heatheria that only the "Chosen One of the Sea" is able to help the three of them destroy Mantene, and the mermaid has been searching for that entity. Little do any of them know that the "Chosen One" is closer than they think, and Marina gets the shock of her life when she learns about the power within her. But when Mantene tricks Marina into unleashing her power upon the Sea Witch, Marina plays right into Mantene's hands when her power is drained from her being. Can Luanne and Heatheria find the Chosen One of the Sea--as well as defeat Mantene's coup to conquer the Known World once and for all?