Luca (voiced by Jacob Tremblay) is a young sea monster - a very cute, mostly human-shaped sea monster. He lives with his family off the coast of Italy - Wikipedia tells me the year is 1959. He herds goat fish and is bored with his pastoral underwater life, but terrified of the surface because his parents have always taught him humans are monsters. But that doesn't stop him being fascinated by human objects that he finds on the bottom of the sea. One day this leads to a friendship with another young sea monster (Alberto, voiced by Jack Dylan Grazer) who also collects human stuff ... and who shows him that when sea monsters go ashore and dry out, they look human. And so begins his summer coming-of-age adventure.
The movie is beautiful, with glorious artwork both above and below the sea. The small town in Italy is particularly lovely. The coming-of-age tale is more or less textbook Pisney-Dixar, but brings an immense amount of charm to the table without getting cloying. Lovely to watch and fun without managing to get near Pixar's best: better than "Onward," on par with "Soul," not as good as "Toy Story 4."