BEYOND THE TIME BARRIER

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. When he returns to the airbase, it appears abandoned, old and deserted. Mystified, he sees a futuristic city on the horizon and heads for it. The major is rendered unconscious and captured.

When Allison wakes, he finds himself in a dystopian underground city known as the Citadel. 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. The Supreme explains that he and his second-in-command, the Captain, are the only two residents of the Citadel who are able to speak or hear. The rest of the inhabitants, including the Supreme's granddaughter Trirene, are deaf-mutes, and everyone except possibly Trirene is sterile. A telepath, Trirene reads Allison's thoughts and indicates to the Supreme that she believes him not to be a spy, as the Captain suspects.

Despite this, the Captain sends Allison to an underground containment area, where a group of bald and violent mutants are determined to kill everyone in the Citadel. As the mutants attack him, Allison overpowers one and demands answers. They claim to be the survivors of a "cosmic plague", and they blame the residents of the Citadel for their problems. The Captain releases Allison and explains that Trirene has convinced the Supreme that he is not an enemy. 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.

After disabling surveillance devices, the scientists explain that Allison has traveled through time to the year 2024. Nuclear fallout suspended in the air damaged the Earth's atmosphere, letting through dangerous cosmic rays in 1971, resulting in the cosmic plague. Even those who fled underground to the Citadel were still afflicted, although not as badly afflicted as those who stayed above ground, and there have been no births in twenty years. 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. Markova explains that the Supreme needs Allison to try to repopulate their society with Trirene's "help". The scientists warn Allison not to trust the Citadel. The Captain in turn warns Allison not to trust the trio.

As Trirene and Allison spend more time together, they fall in love. 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. Markova sets the mutants free to attack the residents. She then demands that she accompany Allison, not Trirene, and to 1973, not 1960. Kruse and Bourman arrive, and Kruse shoots Markova for her treachery. Bourman knocks out Kruse, explaining that Kruse was planning to hijack the aircraft himself. Bourman, however, also intends to use the X-80 to return to his own time. As Allison and Bourman struggle over Kruse's pistol, a stray bullet fatally wounds Trirene before Allison can overcome Bourman.

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. The Supreme directs Allison toward a secret passage out, persuaded by Allison that there is always hope. Returned to his own time, Allison recounts his fantastic adventure in a recorded debriefing. As high-ranking officials visit Allison in the hospital, he is revealed to have aged drastically and is now an elderly man. Allison frantically warns them of the future events, and gives them Trirene's ring, which the Supreme had slipped onto Allison's finger. Secretary Lloyd Patterson says that they have a lot to think about.

Allison allies with fellow time-displaced survivors to escape the Citadel's confines, confronting the mutants and the Citadel's desperate leaders in a bid to change the future.

The climax sees the struggle for survival and the hope of altering a doomed timeline as the central narrative payoff. Ulmer's ending is a meditation on agency and consequence—showing that even in a radioactive, telepathic future, humanity clings to the possibility of redemption, however ambiguous that hope might be.