Marina, a mermaid, falls in love with Prince Justin of a coastal kingdom. After saving Justin from perilous sea creatures, she strikes a deal with the sea witch Hedwig to transform into a human temporarily and woo the prince. Despite their growing bond, Justin mistakenly credits Princess Cecily for his rescue and becomes engaged to her. As Justin recollects Marina's heroism, she reverts to her mermaid form. The star-crossed pair navigates clandestine meetings, thwarting Hedwig's power grab and Cecily's schemes to marry Justin. Assisted by underwater companions like Winnie the seahorse and Anselm the wizard, they strive to unite against all odds, with loyal allies including Bobo the tropical fish and Ridley the sea otter, along with Justin's devoted page, Chauncey.