At the end of the 19th century, in a small village in the Apennines, in a house lost in the middle of a valley, live Alessandro and Pietro, two very different but very close brothers. The two children, aged eleven and thirteen, carefree and full of energy, live like two wild animals, always getting into trouble and spending their days racing carts, diving into the river, and stealing from the cellar. When their mother dies, their father sells his last possessions to buy his sons two beautiful, untamed foals, Baio and Sauro. As adults, while Alessandro feels a growing desire to cross the mountains and go far away, Pietro wants to become a breeder and live with Veronica, the girl he loves.

A group of friends on vacation are attacked by a swarm of killer horseflies. The situation worsens because the flies are attracted to blood and attack their victims, causing them to implode or cause serious injury to feed. The friends must use large amounts of cannabis to repel the insects, discovering that the THC in marijuana is the only substance capable of stopping their rampage.