In preparation for sending a manned rocket into space, American scientists Dr. Quent Brady and Dan Morgan are put in charge of a program that sends various animals and insects into space to test their survival rates. After one of their rockets carrying wasps malfunctions and goes off course, a computer calculates that the rocket is likely to land somewhere off the coast of Africa. Some time later, in a remote part of Africa, Dr. Lorentz and his daughter Lorna perform an autopsy on a native and determine that he died of paralysis of the nerve centers caused by an injection of a massive amount of venom. Arobi, Lorentz' African assistant, then informs him that a monster is believed to be terrorizing people and animals in an area known as Green Hell. Several months later, Brady reads a newspaper account of turmoil in Central Africa caused by gigantic monsters and surmises that the wasps in the missing rocket were exposed to huge amounts of cosmic radiation because an earlier, very minimal overexposure had resulted in the birth of a spider crab twice the size of its mother. Brady and Morgan request a leave of absence from Washington and head for Africa to investigate. In Libreville, equatorial Africa, the territorial agent makes plans for them to travel to meet Dr. Lorentz. Once the safari is ready, Mahri, an Arab, leads Brady and Morgan on the four-hundred mile trek to Lorentz' hospital. The safari battles brush fires, fever, drought and storms, eventually reaching the Lorentz compound where Lorna informs them that her father has not returned from a journey to Green Hell. Later, Arobi arrives with the news that Lorentz has been killed by a monster living in the cauldron of a volcano and gives Brady a giant stinger he removed from the doctor's shoulder. After Brady analyzes the stinger, he confirms that it belongs to a giant, deadly wasp. Although Brady advises Lorna to stay at the hospital, she insists on accompanying him, Morgan, Mahri and Arobi to Green Hell. When the native bearers learn of the destination, they desert, and although Lorna is able to shame several local villagers into helping, they, too, run off when the group comes upon a deserted native village littered with dead bodies. After Brady expresses his concern that the insects may be multiplying rapidly and could eventually overrun all of Africa, he states that they must destroy the queen and her immediate colony. Brady then explains to Mahri that he has brought small, grenade-like bombs, filled with a special explosive, to use against the monsters. As they move closer to the base of the volcano, which shows signs of an imminent eruption, they hear a very loud, buzzing sound. When Brady looks down from a ridge above the volcano, he finds the queen and several gigantic wasps. The four men toss grenades into the bowl, but the explosions only serve to anger the wasps. Lorna and the men are pursued by one of the wasps, the size of a large building, but hide in a cave that it cannot enter. The group escapes through another entrance, and, just as they emerge, the volcano erupts, spewing massive lava flows that destroy all the wasps in the conflagration. Morgan then notes that nature has a way of destroying its mistakes.