Physicist and leading atomic scientist Dr. Cal Meacham [Rex Reason] is flying to his laboratory in a loaned Lockheed T-33 Shooting Star when the engine fails, only to be saved from a crash by a mysterious green glow that surrounds his aircraft. Back at the lab, he discovers an unusual substitute for the electronic condensers he ordered, delivered from a mysterious source. The components feel built from an unknown and nearly indestructible material, and Meacham and his assistant, Joe Wilson [Robert Nichols], waste no time attempting to assemble a curious device called an interocitor. When they complete it, a mysterious man named Exeter [Jeff Morrow] appears on the interocitor’s screen and declares that Meacham has passed a test, inviting him to join a secret research project. Exeter then triggers a beam that causes the interocitor to self-destruct, leaving Meacham both intrigued and unsettled.
Despite his reservations, Meacham is unexpectedly picked up at a fog-draped airport by an unmanned, computer‑controlled Douglas DC-3 and whisked to a remote Georgia facility. There, he encounters an international group of top atomic scientists, including his old flame, Dr. Ruth Adams [Faith Domergue]. Cal is puzzled by Ruth’s apparent forgetfulness of their past, and the pair, along with Exeter’s presence and Brack [Lance Fuller], begins to suspect that they are not being watched as closely as they think. Ruth and Cal, together with Steve Carlson [Russell Johnson], decide to press their questions and test the strange circumstances surrounding the project.
Meacham and Adams flee with Carlson, but danger closes in quickly: their car is attacked, and Carlson is killed in the ensuing assault. They escape in a Stinson 108 single‑engine aircraft, only to watch the research facility and everyone there incinerate in a blaze of radiant light. In a shocking turn, their craft is drawn upward by a bright green beam into a flying saucer. Exeter explains that he and his people are from the planet Metaluna and have been wending a desperate war with the Zagons. Metaluna’s defense relies on a planetary energy field, but the fuel for it—uranium— is running out. The humans have been enlisted to transmute lead into uranium, yet time is rapidly running out. Exeter guides Meacham and Adams toward understanding the stakes of their mission and the precarious balance between two worlds.
Exeter ferries Cal and Ruth to Metaluna, where they sequester them in conditioning tubes to normalize the extreme pressure differences between the planets. Metaluna is under siege by Zagon spaceships, which unleash meteors against the planet, and the ionization layer that protects the world is failing. The Metalunan leader, the Monitor [Douglas Spencer], explains that Metalunans plan to flee to Earth, but they intend to subjugate humanity with a Thought Transference Chamber—an immoral solution Exeter rejects as misguided and dangerous.
Before any mind control can occur, Exeter helps the two Earth visitors escape. He fights a mutant guardian and is badly wounded, but Cal, Ruth, and Exeter flee the dying world in the saucer. As they depart, the ionization barrier collapses, leaving Metaluna to heat into a radioactive sun. The mutant guard—the result of the world’s brutal experiments—engages Ruth, but dies as the pressure difference between the two planets takes its toll on the return journey. Re-entering Earth’s atmosphere, Exeter remains behind to ensure Cal and Ruth’s safe passage, declining to accompany them. The saucer accelerates and, trapped in a fiery blaze, crashes into the ocean, ending Exeter’s sacrifice as the craft explodes while Cal and Ruth head home to a fragile peace on Earth.
