Electronics specialist Dr. Cal Meacham, who is close to discovering how to convert lead and uranium into copious, free nuclear energy, leaves a conference and flies himself home to Los Angeles in a jet. Just before landing, his plane loses power, but he is rescued by a mysterious green light that envelops the plane and brings it safely to earth. Cal and his shaken assistant, Joe Wilson, who has witnessed the landing, return to their lab, where Joe demonstrates the tiny but amazingly strong converter part sent by an unknown company called Direct Electronic Services. Soon, the company sends another package, consisting of an instruction manual made of paper-like metal. Cal orders all the pieces in the book and assembles them according to the directions. The result is a large machine with a screen that instructs them verbally how to turn it on. A man named Exeter, with a large forehead and snow-white hair, appears on the screen and congratulates Cal for passing the test. Exeter invites Cal to join an elite team of scientists, and intrigued, Cal agrees to meet the plane that will take him to Exeter's headquarters the next evening. Although the night is thick with fog, the plane arrives easily, and Cal climbs aboard the empty jet over Joe's objections. Without a pilot, the plane touches down in Georgia, where Cal is greeted by scientist Ruth Adams, who pretends she does not recognize him from a conference years earlier. She brings him to Exeter's lovely offices, where she appears nervous and has a brief, coded exchange with physicist Steve Carlson. Exeter explains to Cal that he has assembled the planet's best scientific minds to find a quick way to produce nuclear power. After Ruth and Cal leave his office, Exeter, who is really an extra-terrestrial serving a superior named The Monitor, receives a message insisting he finish his task immediately. After dinner, Ruth and Steve take Cal on a tour of the labs, where Cal confronts them. Attempting to block the screen on which they can be watched, he asks why they are so nervous, and they reveal that they are the only scientists not to be brainwashed by Exeter. Meanwhile, Exeter's cold assistant, Brack, spies on them and urges his boss to brainwash them, but Exeter replies that the machine takes away its subjects' initiative. The next day, Exeter demonstrates a deadly "neutrino" ray to Cal and asks him not to meet again with Ruth and Steve. Later that week, however, the three make an escape attempt in a station wagon, just as The Monitor instructs Exeter to bring Ruth and Cal with him to their planet, Metaluna. Exeter boards his spaceship while Brack attacks the station wagon with neutrino rays. Steve stops the car to let Cal and Ruth run to safety, but then is quickly killed by a ray. Cal and Ruth race on foot to a nearby helicopter, but once in the air, they are apprehended by Exeter and sucked into the spaceship. In the control room, Exeter, who is dressed in a silver suit along with his fellow Metalunans, asks them to join him peaceably in order to help save his planet, which is being attacked by the enemy planet Zahgon. The shield around Metaluna is powered by atomic energy, but they are running out of fuel and must find a replacement process immediately. Cal and Ruth must don the silver suit and stand in a tube that will condition them for atmospheric changes, and although they are reluctant to trust Exeter, the procedure works as he has promised. Approaching Metaluna, the spaceship is attacked by the same Zahgonian meteors that are pummeling Exeter's planet, and although the ship avoids danger, they can see on the ship's screen that Metaluna is sustaining great damage. They land in the war-ravaged planet's underground society, Exeter points out the destroyed schools and recreation centers on the way to The Monitor's headquarters. There, The Monitor explains that they must abandon the planet immediately, with plans to peacefully relocate to Earth. When Cal blanches at his claim to be superior in intelligence, The Monitor scorns his arrogant belief that Earthlings must be the most advanced peoples of the universe, and Cal replies that Earth's true size is the size of its God. Against Exeter's pleas, The Monitor then insists that the humans be brainwashed, and along the way Cal and Ruth try to run, but are stopped by the sight of a huge mutant half-man, half-insect. Cal then punches Exeter and pulls Ruth outside. Exeter follows and, promising he wants only to help, directs them to the spaceship, but just before they board, another mutant attacks the Metalunan. Cal pulls him off, but Exeter is wounded, and the mutant follows them on board. As they take off, Exeter watches his planet explode into a radiated ball of light, and wishes only that its heat will nourish another planet. They manage to avoid the attacks of the Zahgons, but as soon as they enter the tubes, the mutant attacks again, and only collapses after Cal struggles out of the tube and protects Ruth against it. They soon reach Earth, where Exeter states that he will continue to search for like life forms, and refuses to join Cal and Ruth. The scientists say goodbye and fly their helicopter home, unaware that behind them, a lonely Exeter is hurtling his ship into the sea.