Scientists Dr. Erik von Steiner (Preston Foster), Dr. Steve Connors (Philip Carey) and Carol White (Merry Anders) are testing their time viewing device, drawing enormous amounts of power. Danny McKee (Franken), a technician from the power plant, has been sent to tell them to shut down their experiment. During the test, odd shadows quickly cross the room before the screen shows a stark, barren landscape. Danny discovers the screen has become a portal and steps through.
As the setting is becoming unstable, the others enter the portal to bring him back. The portal disappears, stranding them. Then they are pursued by hostile primitives, ending up in a cave. There they find an underground city - all that is left of civilization in a future devastated by nuclear war.
The year is 2071 A.D. City leader Dr. Varno (John Hoyt) explains that Earth is unable to support life. The residents are frantically working on a spacecraft that will take them to a planet orbiting a distant star. The four time travelers pitch in to help complete the spaceship, but before they can lift off, the degenerate mutant humans break in and destroy the ship and the city.
The three scientists, with help from the power technician, work feverishly with future technology to rebuild their time portal. Along with a few people from the future, they escape back to the present just ahead of the mutants. One person throws an object through the gateway that damages the equipment on the other side and shuts down the portal.
The survivors return to the lab, where they make a horrible discovery. Through some error, they are experiencing time at an accelerated rate; the rest of the world is moving in slow motion. Their only option is to travel to 100,000 years in the future. But the screen is dark and what lies ahead is unknown.
When the last one goes through, the screen flashes on briefly and shows the characters walking in a clearing with trees and grass. The film returns back to the start. The sequence of events of the entire movie is rapidly reshown, and then repeats faster and faster until it abruptly ends without further explanation.