A frustratingly not-terrible action thriller.
"Butterfly" takes place during that brief moment in Spain between the formation of the Republic and the Civil War. A history lesson will be necessary for most viewers, and the movie provides it, explaining that the old order of church, military and monarchy was overthrown by a new leftist government, legally elected, which was then challenged by the right.
The war that followed was like a rehearsal for World War II, with Hitler testing his Luftwaffe and Russia supplying the communist side. The story was more complicated, because the Russians also fought for control of the left against the democratic socialists and the anarchists; George Orwell's Homage to Catalonia tells the whole story from the point of view of an observer who was left-wing but anti-communist.
The point is that freedom flickered before being crushed by big players on the world stage, who ushered in Franco and decades of dictatorship. People dared to admit their real religious beliefs (or lack of them), and to prefer democracy to the king. And as "Butterfly" begins, a 7-year-old boy is preparing for his first day of school in a village in Galicia.
His name is Moncho (Manuel Lozano), and he is frightened, because his older brother sometimes comes home after being beaten. In class, when the teacher calls him to the front of the room, Moncho pees his pants and flees. But then the teacher comes calling. He is a kindly old man named Don Gregorio (Fernando Fernan Gomez), who explains he would never beat anyone. He coaxes the boy back into class, and gently introduces him to the to the world and its wonders. He gives him two presents in particular: Treasure Island, by Robert Louis Stevenson, and a butterfly net. Together, the old man and the boy study nature.