Astronauts returning from a voyage are caught in a time warp and are propelled into a post-Apocalyptic Earth populated by mutants.

In 1957, the American space agency loses contact with the first manned explorer rocket, MRX, and the crew under the command of Dr. Eldon Galbraithe is feared lost. Onboard the MRX, Eldon, engineer Henry Jaffee, radioman Herbert Ellis and scientist John Borden worry about the loss of radio contact with Earth, but after completing their second successful orbit around the planet Mars, plot their return journey home. Shortly after the rockets ignite, however, MRX hits inexplicable and violent turbulence, forcing the ship to accelerate to fantastically high speeds. The crew lapses into unconsciousness as the ship falls into a planet's atmosphere and crashes into a snow covered valley. When the men revive they are surprised to discover that although their instruments register a slight radiation reading, gravity and oxygen also register on the unknown planet. After analyzing the ship's damage, Eldon decides that they are unable to make repairs and should explore their surroundings. Armed with supplies and pistols, the men set off and after traveling several hours in an earth-like environment, discover a cave. Inside, the men come upon a huge web and moments later, are attacked by two giant spiders that they manage to fight off. At dusk, the men set up camp in a clearing, unaware that they are being watched by numerous, grotesque, one-eyed creatures who later attack them. The men kill two of the three attackers and, upon examining the corpses, are stunned by the creatures' similarity to humans. The next morning, the men find a series of markers that they are shocked to realize as gravestones. Upon seeing stones with dates far into the future and many marked "2188," Eldon concludes that they have been propelled into Earth's future, in the aftermath of some cataclysmic disaster. Continuing on, the men are again attacked by the Cyclops creatures and forced to seek refuge in another cave where they find a strange metallic wall that opens, revealing a long, constructed hallway. The men follow the hallway to a room where they are directed by a voice from a loudspeaker to leave their weapons and supplies and proceed into a large, conference-style room. There, to the men's amazement, a group of elderly human men greet them. The leader introduces himself as Timmek and confirms Eldon's conjecture that the men are on the Earth of their future, in the year 2508. Timmek explains that centuries earlier the planet was nearly destroyed by a worldwide atomic war and the radiation fall-out caused strange mutations of insects, animals and humans, the worst of which are the primitive Cyclops people. Those unaffected were forced underground where over the years they created a safe, comfortable world. Timmek then introduces the men to his beautiful daughter Garnet, who escorts the men to a room provided for them. Under questioning, Garnet reveals that her people have remained below ground despite the dissipation of radiation because of their fear of the violent Cyclops people. When John suggests that the scientifically advanced humans could easily destroy the primitive Cyclops, Garnet declares that her people loathe war. Later, John inquires about another woman attending them, Deena, who appears different from the others. Garnet explains that not all the humans above ground remain affected by the radioactive past, that many developed normally only to be killed at birth or early childhood by the Cyclops. Deena grew to adulthood, escaped the savages and was rescued by the underground people. That evening, Hank marvels at the community's development, but John points out that they are limited by their refusal to consider living on the outside. The men also note that the underground men are all weak and elderly, while the woman are vibrant and attractive. Meeting with Timmek and other council members the next day, Eldon asks for assistance in repairing MRX in order to fulfill the men's duty and explore the rest of the planet. Mories, a high council member, expresses unease with the request and points out the men will be besieged by the Cyclops on the outside. When Eldon suggests the undergrounders could help them, Mories steadfastly refuses to become embroiled in violence, but Timmek agrees to consider the matter. Meanwhile John and Garnet grow friendly, as another woman, Elaine, and Deena pursue Herb. After touring the community, Hank reports that the lack of very young children suggests the undergrounders may be dying out. After the council refuses Eldon's request, the men learn from Elaine and Garnet that Mories' great fear of change and violence has paralyzed the undergrounders. Determined to repair MRX, Eldon decides to ask for help in making additional weapons so that they might fight off the Cyclops and return to their ship. Unknown to the men, Mories overhears their plans and reports to Timmek that the men are intent on a brutal takeover. When the men make their request to Timmek and the council, Mories declares they intend to make slaves of the undergrounders and the council fearfully rejects the request. Afterward, Garnet pleads with her father to reconsider and he remains uncertain. Concerned that Timmek may relent, Mories urges council member James, who is keeping the men's guns in his room, to remain steadfast. Mories then breaks into James's room to get the guns and when James attempts to interfere, strikes him down. Mories then plants the guns in the men's room, unaware that Deena is watching. Upon learning of James's violent death, Timmek suspects Eldon and the others and upon finding the guns in their rooms, orders them released to the outside with only their supplies and extra food. Meanwhile Deena seeks to speak with Timmek, but is attacked by Mories. Discovered semi-conscious by the others, Deena relates that Mories has framed the men and Timmek orders his apprehension. Terrified, Mories flees outside where he is attacked and killed by the Cyclops. Timmek and the council apologize to the men and agree to provide help in developing a weapon to drive out the Cyclops. Once the bazooka-like weapon is developed, the men, joined by Deena, go out in search of the Cyclops and engage in numerous skirmishes until John demands a hand-to-hand fight with their leader, Naga. John succeeds in killing Naga, thus frightening and scattering the remains Cyclops. Months later, the men have convinced Timmek's people to join the surviving, non-mutants outsiders and begin human life anew.