Animated feature set on the Italian Riviera during the 1950s and 60s.

Luca Paguro (voice of Jacob Tremblay) is a human-like sea monster who leads a lonely existence under the surface of the water with his parents, who do all they can to protect Luca from the human world. One day Luca meets a fellow sea monster, Alberto Scorfano (Jack Dylan Grazer).

The two boys quickly strike up a friendship and Alberto encourages Luca to leave the sea and explore the land with him. What follows is an adventure that neither boy will ever forget, but as they travel and interact with the human population, Luca's parents set out to retrieve their son.