Call to Adventure (10-13): When Luca intersects with Alberto who drags Luca above the sea line…
Act One end (26-28): Luca overcomes his fears and rides with Alberto on the Vespa, soaring high into the sky, then plunging into the ocean…
Call to Adventure (10-13): When Luca intersects with Alberto who drags Luca above the sea line, instigating the first "transformation" of Luca into human form. Luca is afraid and refuses to stay in the New World, only to return later.
Act One end (26-28): Luca overcomes his fears and rides with Alberto on the Vespa, soaring high into the sky, then plunging into the ocean. Having survived, Luca takes a step toward embracing the courage necessary to prevail on his hero's journey.
The predominant locale for the rest of the story is Human Town. That's where Luca and Alberto run away in an attempt to protect Luca from his parents who want him to go with Uncle Ugo to the deep ocean (this transition happens 34-36).
The Plotline in Act Two focuses on preparation for the triathlon along with various characters searching for sea monsters, humans in the sea, Luca's parents looking for Luca.
Meanwhile in the Themeline, two major characters undergo their own character arcs: Luca becomes more settled in Human Town to the point where he wants to attend school with Giula. As Luca and the girl grow closer, Alberto - fueled by jealousy - becomes a threat to Luca. The rising tension between the three comes to a head when Luca and Alberto begin to fight, the altercation stopped by Giula.
This leads to a big blow-up between Alberto and Luca, and Giula, who finally uncovers the fact that Luca is a sea monster, sending Luca away (94-97), a fitting capstone of the All Is Lost plotline point at the end of Act Two
The centerpiece of Act Three is the triathlon, but the culmination of the story involves humans embracing the sea monsters, even some revealing themselves to be sea monsters in human form. Also, the dispensation of Alberto (who stays in Human Town to work with Massimo) and Luca (who heads off to school with Giula).
Overall, the plot follows three-act structure as detailed in the hero's journey:
Separation
Initiation
Return
In other words, it's a conventional story structure. It's the way in which characters are articulated that provides the story a unique quality.
More in the next article on Characters.