Ulmer's wife Shirley acted as a script editor while their daughter Arianne Arden appeared as a Russian pilot.

It was one of two low budget sci-fi films shot back-to-back in Dallas, Texas by Ulmer (the other being The Amazing Transparent Man, released earlier that year).

In 1960, United States Air Force test pilot Major Bill Allison flies the X-80 experimental aircraft to sub-orbital spaceflight successfully, though he loses radio contact.

Unnerved by his captors' refusal to speak with him, Allison initially reacts hostilely, but he eventually calms down and is brought to their leader, the Supreme.

Sensing his confusion, Trirene shows him historical photographs that help explain the Citadel's history, and at his urging, leads him to the "scapes", two scientists and a Russian woman officer.

The 'scapes themselves are also accidental time travelers: Russian Captain Markova comes from 1973, and General Kruse and Professor Bourman arrived from colonies on other planets in 1994.

Although initially reluctant, Allison joins the 'scapes in a plan to turn Trirene against her people so that he can return to the past and try to change history.

Allison takes Trirene's body to the Supreme, who is distraught over both his granddaughter's death and the doom of his people, now that the last fertile person has died.

Producer Robert Clarke was exhausted from directing and acting in his production, The Hideous Sun Demon, and sought a director for this film.

[5] Clarke's funds originated in Texas, and the backers stipulated that the film be shot there, where motion picture unions had no influence.

[7] Production designer Ernst Fegté employed a triangular motif for the futuristic sets that were filmed in the vacant showground buildings.

Clarke chose Darlene Tompkins over several contenders for the mute and psychic Trirene, including Yvette Mimieux (who appeared in The Time Machine) and Leslie Parrish.

When giving her speech inciting the mutants to revolt, Arianne deliberately used voice inflections similar to Laurence Olivier reciting the St. Crispin's Day Speech from Henry V.[9] American International Pictures (AIP) added footage for the mutant uprising sequence from their film Journey to the Lost City.

Tompkins recalled that the actors portraying the mutants, whose makeup was created by Jack Pierce, taught her how to play cribbage on the set while in costume.

[13] Dave Kehr of the Chicago Reader wrote, "Even on this despairing level of fly-by-night filmmaking, Ulmer's treatment remains resolutely personal, and the film, though visually slack, emerges as something terse, resourceful, and expressively icy.