Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is widely regarded as a masterpiece made during the Czechoslovakian New Wave. Set In the middle ages, we see a woman, Alexandra, perform some sort of naked Pagan ceremony involving a tree covered with skulls and the blood of a dead bird. She lies down in a field of grass with her brother, Adam. Then we see a Viper on a log which turns and crawls towards the couple. Adam jumps as if in pain, grabs his knife and stabs the snake, which is still on the log. We can tell that he was bitten because Alexandra sucks on his left arm to try to remove the venom.
Just like most movie snakes, this one is made to be pure evil. Maybe it's attack is supposed to be the result of Alexandra's Pagan worship, or just a Christian Biblical myth, but it's still hard to believe that an Adder, or any snake, would crawl up to a man in a field and just bite him on the arm. Neither self defense nor a hunting response would explain that. It's simply another movie animal acting like a horror movie monster.
The snake is a real male Adder, Vipera berus, the only species of venomous snake found in Czechoslovakia, where the film was made, and it's obvious that someone really stabbed the snake with a knife, because after being stabbed the snake rears back quickly and strikes at the blade. There's no excuse for hurting or killing animals in entertainment or art, so shame on them.
As a punishment for his sin and to remove the poison in his blood, his father cuts off Adam's arm with an axe, saying that if he dies, Alexandra will also be killed. But he survives, earning the name One-armed Adam and Alexandra continues her naked Pagan tree hugging.