Scientists Dr. Erik von Steiner [Preston Foster], Dr. Steve Connors [Philip Carey], and Carol White [Merry Anders] are testing their time-viewing device, a powerful machine that draws enormous amounts of power. Danny McKee [Steve Franken], a technician from the power plant, is sent to tell them to shut down the experiment. As the device runs, odd shadows sweep the room and the screen flickers to reveal a stark, barren landscape. The screen has become a time portal, and Danny steps through it.
The portal destabilizes; the others rush in to call him back, but the portal vanishes, leaving the four time travelers stranded. Chased by hostile mutants, they seek refuge in a cave that opens into an underground city—the last remnant of civilization in a future ruined by nuclear war.
Within the city, the insiders reveal that the year is A.D. 2071. The leader, Dr. Varno [John Hoyt], explains Earth can no longer sustain life, and the surviving residents, along with their androids [Molly Glessing], are frantically building a spacecraft to reach a planet orbiting Alpha Centauri. Exploratory probes, including a rover-like device, show no habitable planets in the Solar System, and although a Transporter exists, it cannot reach distant worlds.
The mutants, once peaceful, have become fiercely hostile as resources dwindle and now threaten the city’s food supply. The four time travelers are told they cannot join the voyage due to weight, space, and time constraints, but they are allowed to help recreate the time portal so they can return to their own era. Dr. Varno and others assist von Steiner, Connors, White, and McKee as they race to rebuild the device, while the rest of the community readies the liftoff.
Just as the spaceship reaches its final boarding phase, the Mutant [Peter Strudwick] and his followers invade the complex and assault the facility. The spacecraft is destroyed in the attack, and the time portal now shows the travelers’ point of origin. The remaining humans realize their only chance for survival is to step through the portal, taking with them a few people from the future, and sever the mutants’ pursuit by destroying the portal’s link behind them.
Back in the lab, the travelers discover their past selves remain immobile, and they realize calibration mistakes have accelerated time for them, meaning they will die of old age within a single day. Their only option is to travel to the date the portal had briefly been set to before the switch to A.D. 2071—more than 100,000 years in the future. The screen goes dark; when it returns, the travelers are seen moving through a sunlit clearing with trees and grass, signaling that the surface of Earth has become habitable again. Their past selves appear to move at normal speed, while the travelers from the future seem to accelerate, and the entire sequence repeats in a loop with ever-shorter glimpses, until the film ends with a final shot of the Andromeda Galaxy.
