In 1959, Luca Paguro, a timid young sea monster, herds goatfish off the coast of the Italian town of Portorosso. His parents, Daniela and Lorenzo, fearing that humans might hunt him, forbid him to approach the surface. One day, Luca meets Alberto Scorfano, a fellow sea monster child who lives alone above the surface, claiming that his father simply is not around much. Alberto encourages Luca to adventure out of the ocean, showing him that sea monsters turn into humans when dry, but return to their true forms when wet. Luca follows Alberto to his hideout on Isola del Mare, where the boys connect while making and riding a makeshift, fragile Vespa. Upon discovering their son’s actions, Luca’s parents plan to send Luca to live in the deep sea with his uncle Ugo. In retaliation, Luca and Alberto run away to Portorosso to find a real Vespa and travel the world.
The boys run afoul of Ercole Visconti, the local bully and repeat champion of the Portorosso Cup Triathlon, who tries to dunk Luca in a fountain. Giulia Marcovaldo, a young girl, helps them escape. In hopes of winning the money needed for a Vespa, the boys and Giulia form a team for the triathlon, which involves swimming, pasta-eating, and biking. Unable to swim without revealing themselves, Luca and Alberto take on the biking and pasta-eating portions of the race, respectively, while Giulia takes the swimming race. Upon learning this, Ercole vows to beat the group and continue his winning streak.