In a seaside town in Italy, two best friends are sharing the ultimate summer - and the ultimate secret. Luca and Alberto are sea monsters, living deep in the ocean near the seaside Italian town of Portorosso. But when Alberto introduces Luca to life on land, where they can trade fins for legs and look like humans, Luca learns there is much more to the surface than the dangers his parents warned him about.
Together, Luca and Alberto discover a shared love of invention, exploration, and leaping from great heights - and when their desire for a Vespa leads them into Portorosso, they meet Giulia. Giulia quickly befriends the pair and introduces them to the Portorosso Cup, the ultimate triathlon of biking, swimming, and pasta-eating that would award them enough prize money to win their coveted Vespa. But as the three grow closer as friends, Luca and Alberto struggle to keep their real identities secret, and Luca must decide which life he truly wants to pursue.