A stray planet on a collision course with Earth instead takes orbit around our blue marble. What seems like a dead planet suddenly launches a fleet of flying saucers which attack our space fleets.

A planet from another galaxy speeds on a collision course toward Earth, but 95,000 miles away it goes into orbit and sends out flying saucers. News of a possible invasion from outer space creates panic, and Earth's space force leads an attack on the invaders. One of the saucers is maneuvered to land on Earth, and Prof. Benjamin Benson, head of a Pacific island observatory, learns the secret of its electronic brain and uses it to destroy the other saucers. Professor Benson, his assistant Fred Steel, and Fred's fiancée, Eva, receive permission to explore the strange planet to discover the reason for its threat to Earth. They land on the planet, and in a crystal cave, they find thousands of steel skeletons, automatons of an extinct race. Fred and Eva flee the planet, but Benson, intent on discovering the power that brought the planet millions of miles through space, remains behind as rockets from Earth destroy the planet.