After Mark and Valerie witness a pair of giant archeologists get attacked by a man with a laser gun, they hide in a hole where they find a teleporter that sends them to an underground city called Limbo which is populated by giants unknown to those living on the surface. Their leader Taru, explains that if the archeologists discover Limbo, they will start a war which the more advanced underground giants will surely win. Taru wishes to avoid war, but his rival, General Aza, is trying to start one and had sent the gunman to kill the scientists. With the others' help, Mark comes up with a plan to contaminate the soil at the dig site with an explosive chemical that would prevent the surface giants from blasting and discovering the hidden city.