Luca Paguro is a young sea monster who resides off the beautiful coast of Portorosso, Italy, spending his days tending to a lively school of fish. Despite the safety concerns expressed by his overprotective parents, Daniela Paguro and Lorenzo Paguro, who fear the dangers of the surface world, Luca finds himself yearning for adventure beyond his underwater home. One fateful day, he crosses paths with Alberto Scorfano, another sea creature who passionately shares tales of his escapades on land. Alberto entices Luca to join him at his secluded hideout on a nearby island, claiming to live there with his father.
Reluctantly, Luca follows Alberto, and as they reach dry land, they magically transform into humans. Within their secret refuge, Luca comes across a vibrant Vespa poster that sparks an idea; they decide to build their own scooter from the ground up. However, as Luca juggles his thrilling new life, he remains cautious, keeping this secret from his parents, with only his supportive grandmother aware of his double existence.
The tides turn when Lorenzo and Daniela uncover Luca's ventures. In an effort to protect him, they plan to send him deeper into the ocean to live with his uncle Ugo. Heartbroken and compelled to seize his freedom, Luca escapes to Portorosso with Alberto. There, they encounter Ercole Visconti, the town bully and reigning champion of the Portorosso Cup Race. Just as Ercole threatens to expose Luca's identity by dunking him in a fountain, he is saved by a brave girl named Giulia Marcovaldo, who nurtures him and Alberto in her home, where she spends her summers with her skilled fisherman father, Massimo.
Amidst the chaos, Luca's concerned parents venture to the surface in search of their son, splashing water on all the boys at the beach to locate him. Unperturbed, Luca, Alberto, and Giulia hatch a plan to enter the Portorosso Cup Triathlon, hoping to win the cash prize to buy the Vespa they dream of. Each child takes on a different challenge: Giulia leads in swimming, Alberto is tasked with the pasta-eating contest, and Luca learns to ride a bike for the cycling segment. As they train and grow closer, jealousy simmers when Alberto feels left out due to Luca's budding friendship with Giulia. This tension culminates in Alberto revealing their secret to Giulia while Luca remains silent, leading to a rift that pushes Alberto away.
Things escalate when accidental exposure reveals Luca's true sea monster identity, forcing him to flee Giulia's home. Desperate to repair their friendship, Luca finds Alberto at his hideout. Here, amid shared regrets and a heartfelt conversation about Alberto’s lonely vigil marking the days since his father departed, they begin to mend their bond, although Alberto still opts out of the triathlon.
On race day, Luca and Giulia enter without Alberto. They quickly conquer the first two legs of the triathlon. However, as the rain begins to pour during the cycling section, desperate to avoid exposure, Luca takes shelter. Alberto rushes to assist his friend, but in doing so, gets soaked and reverts to his sea monster form. In a courageous act, Luca risks his secret to save Alberto from Ercole’s net, exposing both of them in the process. Giulia, hurt in the scuffle, is carried back to safety by Luca and Alberto. Together, they confront Ercole, revealing the truth to the townspeople of Portorosso: sea monsters aren’t threats but allies.
The film culminates in a heartwarming reunion between Luca and his family, who, along with Massimo, celebrate their adventures. Alberto, having sold their beloved Vespa for Luca’s future, ensures his friend can embark on a new journey to school with Giulia. At the train station, amidst heartfelt goodbyes, Luca embraces his identity as a sea monster once again, watching as the train carries him and his dreams away into the rain.
