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 Alpha Centauri. 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 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 A.D., a date over 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 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. The film 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 faster and faster, leaving the viewer in a time loop until it abruptly ends without further explanation.
Under the working title, Time Trap, production took place in 1963. Director Melchior was unable to secure an adequate budget to fully exploit the potential of the storyline. His work, however, was notable in that later reviewers regarded the production as secondary. "In spite of the low budget, this still looks pretty good thanks to intelligent use of the resources available. The portal the scientists create, as Danny discovers, is more than a mere window on the coming years, because they can actually walk through it and pass through the decades to exist in the future."
At 44 minutes into the film, Forrest J Ackerman appears briefly in a scene depicting several technicians. Ackerman's only line in the movie is "Don't worry. I'm keeping our spacemen happy. Getting things squared away". The joke is that his character works on a device that turns a circular frame into a square frame. At the time, Ackerman was editing a science-fiction magazine titled Spacemen. The Time Travelers was heavily promoted in his magazine on the basis of Ackerman's cameo appearance in the film.
The Time Travelers was a B film, evident by its meagre production values, although both the plot and actors were singled out for mention by critics. Leonard Maltin considered the film, "not bad with a downbeat ending, one of the first American films photographed by Vilmos Zsigmond." The film is lampooned on the Netflix series Mystery Science Theater 3000 in episode 1103.