Later, Alberto reveals that he sold the Vespa they bought with the prize money to buy a train ticket for Luca to go to school with Giulia. Luca is reunited with his parents and they celebrate at Giulia's house. Everyone in the town of Portorosso sees that the sea monsters are not actually bad, especially Massimo, who thanks the boys for helping his daughter. When they get there, Ercole tries to kill Luca with a spear, but Luca puts the bully in his place. Giulia tries to stop Ercole from hurting them but she ends up getting hurt herself. He frees Alberto but also gets transformed. Risking his secret being exposed, Luca runs into the rain to save his friend. Alberto arrives at the scene with an umbrella to help Luca, but he gets wet and is transformed back into a sea monster. It begins to rain and Luca tries to avoid being turned back into a sea monster by stopping his bike under an awning. After completing the first two challenges successfully, they enter the cycling stage. On the day of the triathlon, Luca and Giulia arrive without Alberto. Alberto is still mad at Luca and no longer wants to be a part of the triathlon, but Luca decides to enter with Giulia anyway. After Luca questions Alberto about tally marks on the wall, Alberto reveals that he made the marks counting the days since his father left him there alone. Luca finds Alberto in his hideout sulking. Giulia sends him away, worrying that someone could hurt Luca if his true identity was discovered. Back at Giulia's house, Giulia spills water on Luca, exposing him as a sea monster. Luca does not do the same and instead acts afraid of Alberto, leaving Alberto to retreat to his hideout feeling betrayed. After an argument, Alberto reveals his identity as a sea monster to Giulia. Alberto becomes jealous of Luca and Giulia's growing friendship. When Giulia talks about going to school in another part of Italy where she spends the rest of the year, Luca develops an interest in it.

Over time, Ercole starts to become suspicious of the boys, and Luca and Giulia start to become closer to each other. 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. The triathlon consists of three races: swimming laps, eating pasta, and riding a bike through the town. Luca, Alberto and Giulia decide to enter the Portorosso Cup Triathlon together in hopes of winning money to buy a Vespa.

They begin splashing water on all of the boys there in an attempt to find Luca. Meanwhile, Daniela and Lorenzo are unable to find Luca anywhere, so they decide to go up to the surface to locate him. Giulia takes Luca and Alberto to her house where she lives for the summers with her one-handed father Massimo, an expert fish hunter seeking to kill a sea monster. Just as Ercole is about to dunk Luca's head into a fountain which would expose his identity as a sea monster, he is saved by a young girl named Giulia Marcovaldo. There, they learn of the local bully and repeat champion of the Portorosso Cup Race, Ercole Visconti. Distraught, Luca runs away to the city of Portorosso with Alberto. 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. 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. Luca finds a poster in the hideout showing a Vespa, an Italian scooter, and they soon decide to make their own from scratch. Reluctantly, Luca follows, and they transform into humans on the dry land and make their way to Alberto's hideout. Alberto has been on land many times and has a hideout on an island near Portorosso where he claims he lives with his father. One day, while feeling conflicted about whether or not to go to the surface, Luca meets another young sea monster named Alberto Scorfano. His parents, Daniela and Lorenzo, restrict him from going to the surface out of fear, but doing the same thing day after day ends up making Luca bored.

Luca Paguro is a pre-teen sea monster living off the coast of the Italian city of Portorosso who spends his days herding a school of fish.

Luca the movie