Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is an action/adventure movie with some campy comedy. We often see a twinkle added to one of Doc's eyes to remind us that the movie is pulling our leg a bit. And of course it has lots of snakes - real snakes and animated snakes - or I wouldn't be here.
Doc Savage is a character based on pulp stories first published in the 1930s. That must make him one of the earliest superheroes in pop literature, though his powers seem to be strictly human. He even has a giant igloo in the arctic called "The Fortress of Solitude" which is the same name used years later for Superman's arctic hideout. We are told that Doc is a scientist, an explorer, an inventor, a criminologist, a chemist, and an escape artist.
The movie takes place in 1936 in several locations - the Arctic, New York City, on a yacht in the Carribean, and in the Republic of Hidalgo in South America. Doc lives in a penthouse in New York City, drives a bronze sports car with his name on it and flies a bronze helicopter and a bronze plane with his name on it. Even his belt buckle has his name on it. He must have trouble remembering who he is. Way ahead of his time he even has a bronze remote-controlled airplane and the coolest telephone answering machine system I have ever seen that uses a microphone, a speaker, and two turntables, one of which can record to disc. We see it when he gets a message that would have led to the sequel, though a sequel was never made.
Doc has many skills - he can read his friend's thought waves from thousands of miles away, he can pull a bullet out of a wall with his fingers, he's a master at mixed martial arts, and all women find him irresistable. He also has some cool toys: his cigarette lighter is a laser, his watch is a remote control for his safe and an alarm for his penthouse, his tuxedo is bulletproof, and he has compact scuba gear that can be hidden under a tuxedo. He's a lot like Batman or James Bond, relying on technology as well as his wits and physical strength and abilities, rather than on magic superpowers. He also travels with a squad of five war buddies called "The Fabulous Five" that consists of men who are all eminent in their fields - a lawyer, an engineer, a geologist/archeologist, an electrical wizard who has a stun gun, and for comic relief - a slow-witted chemist who always carries his little pet piglet named Habeus Corpus.
After Doc escapes a mysterious red-fingered native man with a green snake painted on his bare chest who tried to kill him, Doc and the Fab Five fly to South America where Doc's father recently died, supposedly of a tropical disease, though Doc suspects murder. A rich villain called Captain Seas tries to stop them more than once, but they trick his airplane pilot-assassin and escape his henchmen.
The first time we see any snakes is in the room of the failed pilot-assasin. Previously we saw Captain Seas tell Kulkan, the medicine man of the Quetzamal people, to get rid of the pilot. We see the pilot lying on his bed when several magic bright green snakes suddenly appear floating around the air, biting him to death. Later more bright green snakes show up in Doc's room, but he manages to fight them off with furniture and then blows them out of the room with a small room fan. Why they didn't return is a mystery not dealt with.
In Hidalgo, Doc meets a secretary named Mona who offers to guide him and the Fab Five to the mysterious tribe, thought to be extinct for 500 years, that deeded their land to his father before the bad guys destroyed all copies of the deed. When Doc and the gang are camped in the jungle, Mona goes to get some water to cook with. To her left we see a large green snake crawling on a tree. We don't know if the snake was put there to kill her or if it is just out for an afternoon crawl, but when Doc sees it he picks up a large cooking knife and throws it straight into the snake's neck. We can add expert knife thrower to his abilities. One of the Fab Five remarks that the snake is identical to the green snake that was painted on the chest of the failed assassin in New York City. Later we see that same snake painted on the chests of the Quetzamal men.
Doc milks some venom from the dead snake's fangs and later we see him in his tent, where he somehow has found a bunch of laboratory equipment, making an antidote out of the venom that comes in very handy later.
Doc, Mona, and the Five travel down the secret path down into the Valley of the Vanished where the Quetzamals live, but they are discovered and everyone except Doc is captured and imprisoned in a cave behind a carved stone door. The Quetzamal chief tries to stop Kulkan from abusing their cultural heritage that he calls The Green Death, which turns out to be those magic floating snakes. The chief is then thrown into the cave for that and because he opposed Captain Seas, who with Kulkan's help has enslaved the tribe to force them to remove vast amounts of molten gold that bubbles up from the ground and give it to the Captain.
We see a Quetzamal man put a long noose into a pit full of green snakes and pull one of them out. (It's a nice noose with a loop on the head end that is closed tight when you pull the other end of the string on the tail end.) He holds the snake up for Kulkan, who milks its green venom into a container then puts it on the tip of a long pole. He puts the envenomated tip of the pole into a hole in the stone door to kill the prisoners.
We see the magic snakes appear from the tip of the pole inside the cave where they bite the captives. But Doc saves the day, gets the door open, and hands them a syringe filled with the antidote to the Green Death venom that he made from the snake he killed earlier.
When Kulkan and his men see the magic Green Death snakes come out of the cave they panic and run. Kulkan accidentally falls into the pit full of green snakes. We hear him screaming but all we see is his hand trying to escape as we hear snakes hissing and striking again and again. Finally we see only his hand lying on top of the pit after he is killed by his own magic.
The Green Death magic snakes were made with some very basic special effects. Somebody just painted them on the film in a form of crude but effective animation. Nobody ever explains what they are, but we can tell they're magic snakes from their shapes as well as from the constant hissing and striking snake sounds we hear. Later when we see the real green snakes and find out how they are made it's more obvious that they're magic snakes.
The live green snakes we see in the snake pit are snakes of several kinds that were simply painted green. The green snake on the tree is also painted green, which is most obvious in the venom-milking close-up. I'm pretty sure they painted a live rattlesnake whish was then actually killed with the knife because you can see the snake react to the knife through its head. The snake that we see pinned to the tree with the knife when the actors are in the same space with it is a fake prop snake or a dead snake that isn't even painted the same color green, it's more blue. When we see the real live snake by itself on the tree it was most likely filmed at a location with only the animal wranglers and camera crew present. They wouldn't have put actors that close to a live rattlesnake.