Marina, a mermaid, falls in love with Prince Justin, a human prince who belongs to a kingdom near the sea. In a great battle, while Justin was drowning, Marina saved him from a giant Cyclops and a vicious sea serpent. To win Justin's heart, she makes a deal with the evil sea witch, Hedwig, promising to transform into a human for a limited time. Although their love blossoms during their amorous meetings, Justin gets betrothed to Princess Cecily, assuming that she was the one who rescued him from Neptune's grip. Eventually, Justin recalls Marina's bravery, but it's too late as she transforms back into a mermaid. To find a way to be together, they must avoid Hedwig's intentions to rule the kingdom and also address Cecily's attempts to unite with Justin. Fortunately, with the help of their aquatic friends, such as Winnie, the seahorse, Bobo, the tropical fish, and Ridley, the sea otter, they manage to tackle their adversities. Moreover, Anselm, the wizard, and Chauncey, Justin's page, also lend a hand.