One day, Luca meets another young sea monster named Alberto Scorfano, who has been on land many times and has a hideout on Isola del Mare, an island near Portorosso where he claims he lives with his father. Luca discovers that he can transform into a human when completely dry, but turns back to his true form when wet. Luca and Alberto make their way to Alberto’s hideout, where Luca finds a poster in the hideout showing a Vespa, and they soon decide to make their own from scratch. All the while, Luca tries to keep his now double-life a secret from his parents with only his grandmother being clued in and keeping it a secret.
Lorenzo and Daniela soon discover what Luca has been doing and decide that to keep Luca safe, he should be sent deeper into the ocean to live with Lorenzo’s brother Ugo. Distraught, Luca runs away to the city of Portorosso with Alberto so that they can find a Vespa and travel the world. There, they run into Ercole Visconti, local bully and repeat champion of the Portorosso Cup Race, but are saved by a young girl named Giulia Marcovaldo. Giulia takes Luca and Alberto to her house where she lives for the summers with her father Massimo, an expert fisherman. Meanwhile, Luca’s parents soon discover that Luca is missing and head to the surface to find him.
Luca, Alberto and Giulia decide to enter the Portorosso Cup Triathlon together in hopes of winning money to buy a Vespa. The triathlon consists of three races: swimming laps, eating pasta, and riding a bike through the town. Each child decides to take up an individual task: Giulia takes up swimming, Alberto takes up eating pasta, and Luca takes up riding a bike after learning how.