Scientists Dr. Erik von Steiner, Dr. Steve Connors, and Carol White are testing their time-viewing device, drawing enormous amounts of power. Danny McKee, 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, where they find an underground city - all that is left of civilization in a future devastated by nuclear war.
The year is 2071 AD. City leader Dr. Varno explains that Earth is unable to support life. The residents are frantically working on a spacecraft that will take them to a planet orbiting Alpha Centauri. The four time travelers, told they may not join that space voyage due to limited space on the ship, are allowed to work on recreating their time portal to return to their time. Before the colony residents can lift off, the mutant humans break in and destroy the ship and encroach on the city.
Dr. Varno determines that now the only hope is the time portal, so he commits the city's remaining resources to help von Steiner, Connors, White, and McKee rebuild the time portal with future technology. As they feverishly work, the mutants are invading the colony; along with a few people from the future, the four travelers escape back to the present just ahead of the mutants. One person throws an object back through the portal that damages the equipment on the other side and shuts down the portal.
The survivors return to the lab, where they make a strange discovery. Their past selves are still in the lab, yet to pass through the portal, but they appear frozen. Through some error, the travelers are experiencing time at an accelerated rate; the rest of the world, including their past selves, is moving in extremely slow motion. Their only option is to travel to the date the portal had briefly been set to before being set to 2071 AD, a date over 100,000 years in the future, but the screen is dark and what lies ahead is unknown. They quickly cross the room, casting their shadows.
When the last one goes through, the screen flashes on briefly and shows the travelers walking in a clearing with trees and grass with the surface of the Earth habitable again and people able to build a future there. In a scene cut from the MST3K episode, the film then shows their past selves moving at normal speed again, repeating the actions seen at the beginning of the film. The sequence of events of the entire movie is rapidly re-shown, and repeats with ever briefer and fewer clips, leaving the viewer in a time loop until it abruptly ends without further explanation.
- Preston Foster
- Philip Carey
- Merry Anders
- John Hoyt
- Dennis Patrick
- Joan Woodbury
- Delores Wells
- Steve Franken
- Berry Kroeger
- Forrest J Ackerman makes a brief cameo as technician turning circles into squares, which was his first credited role.
- The Time Travelers served as an inspiration for the TV series The Time Tunnel.