A spaceship from a previously unknown planet lands in the Scottish moors, bringing a humanoid alien to Earth near the observatory of Professor Elliot (Raymond Bond), just days before the mysterious Planet X will pass closest to our planet. When the professor and his friend, American reporter John Lawrence (Robert Clarke), discover the spaceman, they help it when it is in distress and try to communicate with it, failing in their attempt. They leave, and the alien follows them. A colleague of the professor, the unscrupulous and ambitious scientist Dr. Mears (William Schallert), discovers that the humanoid speaks in musical tones and tries to force from it the metal formula for its spaceship. He shuts off its breathing apparatus and leaves the spaceman for dead, telling the professor that communication was hopeless.
Soon, Lawrence discovers that the alien is gone, as is the professor's daughter, Enid (Margaret Field). Tommy, the seaside village's constable (Roy Engle), reports that others are now missing as well. Lawrence takes the constable to the site where the spaceship had landed, but it is no longer there. With more villagers now missing, including Mears, and with the phone lines suddenly dead and the village in a panic, they are finally able get word to Scotland Yard by using a heliograph to contact a passing freighter just off the coast.
When an Inspector (David Ormont) and a sergeant fly in and are briefed on the situation, it is decided that the military must destroy the spaceship. Lawrence objects that doing so will also kill the people who are now under the alien's control. With the planet due to reach its closest approach to Earth at midnight, Lawrence is given until 11:00pm to rescue them. He sneaks up to the alien ship and learns from Mears that the spaceman intends to use its ship as a wireless relay station in advance of an invasion coming from the approaching planet, which we also learn is a dying world. Lawrence orders the enthralled villagers to leave and attacks the alien, shutting off its breathing apparatus, then escapes with Enid and the professor. Mears, however, returns to the spaceship and is killed when the military opens fire and destroys it, shortly before the planet is nearest Earth. No invasion happens and the mysterious Planet X slowly exits the solar system for deep space.
