|This takes place near the Rio Negro in the Amazon basin in Brazil. We see a priest, Father Mahoney, who has a parish of native people in the jungle. He has dreams of building a hospital and a school, but after his supervisor Father O'Reilly visits him and tells him that's not possible, he becomes disillusioned. He tells O'Reilly that naked women bathing in the river are there to tempt him, and eventually he succumbs to the temptation.
Father Mahoney leaves, paddling his canoe again on the river. He hears unusual sounds then silence and makes a concerned face. He reaches for his cross but realizes that he no longer has it then immediately a large snake raises up out of the water and attacks him. We see him struggle as we hear water splashing and bubbling sounds, then we see only snake, bubbles in the water, and Mahoney's face underwater where the snake dragged him and suffocated him. We see his empty canoe on the water, then immediately, in a flash forward, we see the woman holding their baby in the river. She takes the cross from around her neck and puts it around the baby. Then we flash forward again to see the boy maybe 8 or 10 years later.
When Father Mahoney comes to find out about O'Reilly's disappearance, the native people tell him that Mahoney paddled on the river and at the place where the water turns black a dolphin appeared then changed into a beautiful woman. She lured him into her bed then bore a child and now lives at the edge of the black water.
The rest of the movie deals with the boy, called the dolphin boy because he rides the dolphins that protect him from dangers such as caiman (a type of alligator.) But the dolphins can't protect him from the greedy and violent men who discovered gold in the area and kill his mother then kidnap him and bring him to the city. O'Reilly finds the boy and knows that the cross around his neck proves he is the child of Mahoney. He calls him Lazaro because he has risen from the dead and sends him to an orphanage.
The snake scene is very short and unexpected enough to be shocking. The snake we see is not real, it's only a simple prop, but the effect is real enough. We only see the snake in brief close-ups, but that's better in my opinion than seeing too much of a fake snake, or worse, a bad CGI snake. The snake is probably supposed to be an Anaconda, the largest snake in the area. The snake's action is unrealistic, but in the movie's world of magical realism where a dolphin turns into a woman, I think it's more symbolic than real. It is only meant to represent what happens to a priest who broke his vow of chastity and lost his faith. The cross he loses obviously symbolizes the protection of his faith, and it continues to represent protection in the rest of the movie for Lazaro and then for a friend he gives it to.