Somewhere in the Mediterranean Sea, there lives a young boy named Luca with his family under the ocean. Luca is a "sea monster", a humanoid fish that quickly grows bored from his monotonous life tending to his family's underwater livestock, but his family forbids him from exploring the surface world out of fear that he might get himself killed by a fisher.
Then one day, he meets Alberto, an older sea monster boy who had regularly gone to the surface world, who convinces him to come with him to see the sights. Finally on the surface, Luca discovers that sea monsters look just like regular people when they're completely dry and Alberto takes him to his hiding place where Luca finds out about Vespas, believing them to be height of all joy. With both wanting to have a Vespa to call their own, they go to Portorosso to get one, only to find out that the town is extremely hostile towards sea monsters and they need to keep a low profile. They still make a friend in the cheerful Giulia, a human daughter of a local fisherman, who tells them about an annual competition in town that could net them a Vespa by winning it fair and square.
So the three friends prepare for the competition as Luca's overprotective parents have also gone to the surface to find their son, unfortunately, they have no idea what he looks like as a human.
