Mark,
Valerie, Fitzhugh and Barry see giants Dr. Krane and Mylo doing some drilling. A
robedfigure, General Aza, blasts a boulder with a strange weapon, flattening Krane and Mylo. Fitzhugh is knocked unconscious, and the others carry him away. Krane and Mylo, blaming them for the explosion, follow. Taking refuge in a rocky passage, Mark discovers that they are on a grid-Iike floor. With a flash of blue light, they are transported to the underground city of Limbo. Taru, the prime magistrate, believes they are spies for the surface dwellers, who nearly annihilated his people and forced them to create an underground civilization. Mark tells him one of his people blasted a rock above, and Taru, saying it is against the law for them to go above ground hides them and calls for Aza, who is his opponent in an election. Taru tells him that one of their people used their secret weapon above ground. Aza pretends to be shocked and, while Taru's back is turned, poisons his drink. Aza leaves, and Mark warns Taru about the poisoned drink. Promising to let them go, he restores Fitzhugh to consciousness. He has a sprained ankle, and Taru agrees to let Mark get his friends to help. On the surface, Mark runs into Steve and Dan coming to look for them and they overhear Krane and Mylo talk of blasting. While they phone for explosives, Steve and the others reach the passage. Leaving Dan to watch, Steve and Mark step on the grid and are transported below. Mark tells Taru of the plans to blast. Aza approaches and finds Valerie and Fitzhugh. After he leaves, Taru says he can't release Valerie and Fitzhugh now that they've been discovered and transports Steve and Mark above ground. Steve says they must find a way to prevent the blasting and radios Betty and Barry to bring them the electrical kit. Meantime, Taru mobilizes Valerie and Fitzhugh. Mark cuts into the field shack phone, and Steve, pretending to be a reporter, talks to Mylo. He learns that the only thing that would prevent the blasting would be the presence of Omicron 4, a chemical which could cause an underground fire. Steve returns to Taru, who provides him with some Omicron-4. Aza approaches, and Steve is in danger of being discovered. Taru sends Aza away and returns Steve to the surface. He and the others hope to plant the Omicron-4
In the drill hole, but Krane and Mylo won't leave the site. Dan signals that the truck carrying the
explosives is approaching. Betty volunteers to draw Krane and Mylo away and is caught in the
process. Unable to reach the site because of a huge horned toad, Steve and Mark go to the shack, where Betty is being held. She tells them the archeologists are running an Omicron 4 test, in the shack, and Steve pours some into their test tube. Meantime, Taru wins the election and, learning that Aza knew about the blasting, says the little people are going to stop it. Aza immobilizes him and goes above ground. Seeing him, Steve leads Krane and Mylo to him. Aza is accidentally Killed in a fight with Mylo, who then discovers the Omicron-4 in their test tube. Krane calls off the entire operation, and Mark goes below to report their success to Taru, who has been mobilized by Valerie and Fitzhugh.