The movie opens showing a rocket launch facility on a desert landscape. The narrator, Dr. Quent Brady (Jim Davis) tells us, "This is the age of the rocket, the jet, atomic power. When man prepares to reach for the stars. But before he dares to launch himself into space, there is one great question to be answered: What happens to life in the airless void above Earth's atmosphere? Will life remain untouched, unharmed by its flight thru space? Or will it change into, what? There was only one way to find out and we were working on it." Brady hands Dan Morgan (Robert Griffin) a guinea pig. The animal and insect life is loaded into a V-2 rocket. It is launched, and title and credits are shown over scenes of rockets taking off.Brady and Morgan talk about their successful launch into space. The radar operator (an uncredited Frederic Potler) announces he is losing contact. It was over normal velocity. Brady and Morgan walk upstairs to the computer room. Morgan reads figures into the computer. The answer comes only as an approximation. They refer to a globe and determine the rocket would land just off the coast of western Central Africa.Natives bring a body on a stretcher to the hospital of Dr. Lorentz (Vladimir Sokoloff). Arobi (Joel Fluellen) answers Lorentz's questions about what happened to the dead man. Arobi reports the man followed a trail into Green Hell. He admits he was with his blood brother, but the density of the jungle obscured his seeing his friend, but he responded to the man's screams. He saw nothing. The other men turn their heads when asked what killed the man. An autopsy reveals the cause of death--paralysis of nerve centers. Lorna Lorentz (Barbara Turner) asks if it could be snake venom. Her father says it is possible, but the dosage too massive. Arobi tells Lorentz that large and small animals are fleeing from Green Hell, even the birds have left the area, all in the last week or two.A group of animals are drinking at a watering hole. A large, buzzing, insect comes into the scene. The animals flee before it. The insect is a wasp, the size of a truck. It has compound eyes, wings too small to allow it to fly, and two horn-like projections like elephant tusks. It resembles a beetle more than a wasp. A tribe of natives run away as it climbs over a hill towards them. Brady sees a newspaper article entitled, "Central Africa in Turmoil," and shows it to Morgan. Brady tells Morgan that the rocket landed in Africa six months ago. Brady shows Morgan the side effects on test animals to exposure to cosmic radiation. Short exposure causes coloration changes and behavioral changes. A baby spider crab is four times larger than its mother. The baby was unborn when its mother was exposed: a genetic mutation. Brady wrote a report and his request for a leave of absence was granted.Brady and Morgan fly to Africa on a TWA Lockheed Super Constellation four-engine aircraft. They meet in Libreville, Equatorial Africa with the Territorial Agent (an uncredited Tim Huntley). The agent reports that there are problems in the interior and dangerous. The area around Mt. Virunga, also known by the natives as Green Hell, is the area in question. The agent suggests they visit Dr. Lorentz before going to Green Hell. They don't realize that Dr. Lorentz and Arobi have taken a small group to do a little exploration of their own. Lorentz and Arobi hear the strange buzzing sound. Arobi recognizes it, and we see elephants stampede. Two bearers abandon their loads and leave Lorentz and Arobi on their own. One of the bearers is speared by the wasp in a clearing. His screams attract Lorentz and Arobi. The other bearer attacks the insect with a spear, but is grabbed and killed by a claw around his neck. Lorentz and Arobi find no signs of the men, but spot an enormous footprint they do not recognize. Lorentz continues along a path towards Mt. Virunga while Arobi remains behind.It has been ten days, and Brady and Morgan gather supplies including hand grenades for their safari. Brady makes some notes in his journal. They hired an Arab guide, Mahri (Eduardo Ciannelli) and the next day start their 400 mile trek to the hospital of Lorentz. It would take about a month. Two weeks into the journey, Brady began to feel a sense of dread and foreboding. They hear war drums in the distance. Brady's party was attacked by hostile natives. A fire set in the brush prevented the horde from reaching Brady's bearers. They changed course to a longer route but ran low on water. They find a watering hole, but Mahri orders them not to drink. He spots the reason, the vultures are attracted to a recently dead lion. One man who drank screams and dies. The water has been poisoned. They continue on despite dehydration; a rainstorm brings relief. A few more days of walking and Brady falls ill. He is carried on a stretcher. He recovers at the Lorentz hospital. Lorna Lorentz pronounces Brady as fever free. They learn that Dr. Lorentz left searching for the source of all the problems. Arobi returns to the hospital and reports that Dr. Lorentz is dead. He died along the valley path to the volcano. He shows the two scientists a fragment that was in the doctor's shoulder. Brady determines it is a wasp stinger and Morgan discovers it contains venom. Brady, Morgan and Mahri prepare to go into Green Hell to do something about the giant insects. Arobi tells Lorna they must do something because if the monsters breed and get out of Green Hell, the world is endangered. Lorna arranges for local villagers to assist and she informs Brady she will accompany them on their journey.The next morning they start the trek to Mt. Virunga. Along the route they spot a village full of dead natives. The bearers abandon their loads and run. The remaining members of the safari examine a few of the bodies, but continue on. Large, strange footprints indicate the village was attacked. Brady recognizes the footprints as belonging to wasps. The volcano shows early signs of eruption. That evening Brady briefs his party on wasps, their social organization, and how rapidly they reproduce. The destruction of the queen and colony is their objective. Morgan briefs Arobi and Mahri on how to handle the grenades. As Brady is making his notes by the fire they hear the buzzing of the giant wasps. Arobi also recognizes the sound. A giant snake engages with a wasp and is dispatched with the insect's venom. The wasps surround the camp, but don't attack. Brady speculates, "Maybe they don't like fire. Let's find out." They pour kerosene on the fire.A search the next morning shows no signs of the insects, but Mt. Virunga shows an increase in activity. It is erupting a larger ash plume. They enter a valley (Bronson Canyon, Griffith Park, Los Angeles, California) and hear the insects. They drop their packs and rest only briefly. They walk towards the sound of the wasps. Brady scouts alone. He spots a lone forager, then the queen and colony. They get the grenades and attack the colony. Unfortunately the explosives have no effect on the insects. They run into a nearby cave to elude one chasing them. Brady tosses a grenade into a box of same. The explosion closes the cave entrance. Arobi lights the torches. They must find a way out of the now sealed cave. They inch along the cave wall until they come to a divide. Each of the two tunnels is explored. Brady inches along the floor in a small volcanic tube. Morgan and Mahri explore a larger tunnel and discover an exit. Brady returns and the five take the larger tunnel to outside. By now Mt. Virunga erupts with lava. The hot magma flows down into the area populated by the wasps and they are destroyed. Brady announces, "Well, it took a volcano to do what we failed to do." Morgan adds, "Nature has a way of correcting its own mistakes." We close with a scene of a lake of steaming molten lava.