Luca is a fearless sea monster who spends his time exploring the surface world and teaching Alberto everything he knows. One day, the older boy tells Luca that his dad left him a long time ago and isn't coming back. After Luca asks why, Alberto reluctantly reveals that he used to count the days until he realized it was hopeless and stopped.
Despite their differences, the two become good friends. They both share a passion for discovering the outside world and dream of buying a Vespa that could take them to all corners of the globe. Alberto eventually forms a close bond with Massimo, who becomes his surrogate father figure. He even sells the Vespa he's always dreamed of to give Luca a ticket to Genoa.
When Luca and Alberto sneak into the human town, they are amazed by their surroundings. However, they soon get disrupted by a self-absorbed kid named Ercole Visconti who taunts the boys with harpoons. Luca tries to convince him to leave them alone but fails. Giulia, a local fish delivery girl who is also friends with Massimo, saves the pair.
As they head back to the ocean, they run into Massimo and Luca's parents who warn them that there are monsters on the island. Alberto is sad to hear this and cries into the water. As he does, he sees the tally marks, 383 of them, and remembers his dad. As they swim away, Luca and Giulia wave goodbye.