In a beautiful seaside town on the Italian Riviera, a young sea monster named Luca shares a life beneath the waves herding fish with his family, dreaming of the world above the surface. Curious about life on land, Luca's world turns upside down when he meets Alberto, another young sea monster with a passion for human artifacts. Alberto introduces Luca to the wonders and possibilities of life above the sea. When they venture onto land, they transform into human boys, a magical secret they must keep from the townsfolk, who fear and despise sea monsters.

As summer unfolds, Luca and Alberto immerse themselves in the human world, relishing in local delights like gelato and pasta and cultivating a friendship with a spirited girl named Giulia. The trio embarks on an unforgettable adventure to win the town's annual race, the Portorosso Cup, hoping to secure a prize that could change their lives forever. However, as they work toward their goal, they must navigate through the local customs, evade Giulia's suspicious father, and face the town bully. As they press on, Luca's fear of his true identity being discovered looms overhead, threatening the new life and friendships he's found on this exhilarating excursion above the sea.