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 cross the room and the screen shows a stark, barren landscape. Danny discovers the screen has become a "time portal" and steps through.

The portal becomes unstable, and the others enter it to call him back. The portal disappears, stranding them. Pursued by hostile mutants, they seek refuge in a cave, which they discover leads to an underground city – all that is left of civilization in a future devastated by nuclear war.

They discover that the year is A.D. 2071. City leader Dr. Varno explains that the devastated Earth is now unable to support life, and that the residents, along with their androids, are frantically working on a spacecraft which will take them to a planet orbiting Alpha Centauri. Exploration, including by probe that looks like the Mars rover, has shown that no planets in the Solar System are habitable for humans. And while the future people have a Transporter it is of no use to reach distant planets.

The mutants, once peaceful, have become increasingly hostile as Earth's resources dwindle and now intend to obtain the city's food supply. The four time travelers, told they may not join the space voyage (due to weight, space, resource, and time restrictions), are allowed to work on recreating their time portal to return to their own time. Dr. Varno and others help von Steiner, Connors, White, and McKee race to build another time portal, while the rest of the community readies the spacecraft for liftoff. However, the mutants invade the complex and attack just as the spaceship is about to leave. The spacecraft is destroyed while trying to take off. The time portal shows the time travelers' point of origin, and the remaining humans realize their only chance for survival is to enter it. Along with a few people from the future, the travelers escape back to their own time. They throw objects back through the portal to damage the equipment on the other side and shut down the portal so the mutants cannot follow them.

The survivors return to the lab, where their past selves are still present but immobile. The travelers realize that due to mistakes in their calibration of the portal, they are experiencing time at an accelerated rate, and will therefore die of old age by the end of the day. Their only option is to travel to the date the portal had briefly been set to before it was switched to A.D. 2071. That previous date is over 100,000 years in the future, but the screen is dark; what lies ahead is unknown. They cross the room, casting the shadows seen before the portal appeared.

When they enter the portal, the screen flashes on and shows the travelers walking in a sunlit clearing with trees and grass; the surface of the Earth is habitable again. Their past selves are then shown moving at normal speed again, repeating their actions at an ever accelerating rate; the events of the entire movie rapidly cycle, repeating with ever briefer and fewer clips, implying a time loop, until the film ends with a shot of the Andromeda Galaxy.

Production began in 1963 under the working title Time Trap.^{[citation needed]} Director Melchior was unable to secure an adequate budget to fully exploit the potential of the story line.^{[citation needed]} In some scenes conjuring tricks are used to compensate for the lack of visual effects; for instance, in one uncut shot, a mask-wearing actor playing an android appears to have his head removed and replaced by another, before walking away.^{[citation needed]}

At 44 minutes into the film Forrest J Ackerman appears briefly in a scene depicting several technicians. Ackerman's only line is "Don't worry. I'm keeping our spacemen happy. Getting things squared away." The joke is a self-referential sight gag; his character is working 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.

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".^{[3][N 1]} It was lampooned decades later in the Netflix revival of Mystery Science Theater 3000.

In an early 2010s retrospective review, Graeme Clark of The Spinning Image wrote, "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."^[5]

• List of American films of 1964
• List of apocalyptic films
• List of films featuring time loops

• The Time Travelers at IMDb  
• The Time Travelers at AllMovie
• The Time Travelers at Rotten Tomatoes
• The Time Travelers at the TCM Movie Database
• The Time Travellers at Trailers from Hell