An unusual V-shaped formation of meteorites crashes into Cornwall, drawing a team of scientists to investigate. The mission is initially led by Dr. Curtis Temple [Robert Hutton], who is recovering from a car accident and bears a silver plate in his skull, and is ordered by his doctor not to go. Instead, Temple entrusts the mission to his colleague and lover, Lee Mason [Jennifer Jayne].
At the site, the meteorites prove to be striking in both form and hue—sharp, blue, and unlike any rock, and they secretly house creatures that exist as pure energy. A geologist tries to carve off a piece, and the rock responds with a flash of light and a piercing screech as the aliens seize control of the scientists’ bodies and minds.
Concern rises when it becomes clear that Lee has been requisitioning millions of pounds’ worth of equipment—including weapons—so Temple, defying medical orders, decides to visit the site himself. Before he leaves, a fellow scientist confirms the meteorites came from the Moon, a revelation that fuels the urgency around the mission.
The site soon resembles a fortified outpost, complete with armed guards and a 10,000-volt electric fence. Lee bars Temple from entering, and the situation grows more perilous as an Internal Security agent, tailing Temple, must obtain permission to report the full scope of the crisis. He enters a phone box to make the call, only to stagger out covered in red spots and collapse; a doctor who tries to help him dies after touching him, and the crowd following him succumbs one by one. The press dubs the obscure illness the Crimson Plague, and while no cause or cure is found, authorities devise a secret method for disposing of the bodies.
After a nocturnal rocket launch at the site, Temple resolves to stop a second attempt. A skilled marksman, he returns the next day with a sniper rifle and destroys the power generator that sustains the camp, climbing over the now-deactivated fence to gain entry.
Inside the aliens’ headquarters, Temple—an expert in unarmed combat— defeats one of their embodiments. In an underground complex, he discovers the frozen remains of the plague victims. He is captured and imprisoned in a cell, but escapes by using the door as cover, leaping the alien who comes to kill him. He rescues an unwilling Lee, knocking her unconscious and carrying her away.
Temple brings Lee to the home of his ally Farge [Zia Mohyeddin], where he learns that the silver plate in his head somehow shields him from possession. With Farge’s help, Temple melts the silver cricket trophies to fashion a protective, colander-like helmet. Farge uses ultraviolet light to exorcise Lee’s alien, and she later remembers nothing of that time—though she can imitate an alien and drive back toward the site with Temple and Farge hiding in the car’s rear.
Lee is also protected by a silver helmet, and the trio hides inside a rocket just before liftoff. Their escape is short-lived, however, as they are soon discovered and brought before the Master of the Moon. He explains that the government’s plan to deal with the plague victims is to shoot them toward the Moon, where the aliens—creatures of pure energy—are using human help to repair a spaceship that crashed on their home world.
The aliens hope to return home to die after years of travel, and the frozen victims Temple found are not truly dead. The Master assures them that once the repair is complete, the victims will be restored. Yet the aliens are determined to plunder Temple’s knowledge by removing his silver plate. Farge leads a revolt and saves him, preventing the extraction.
In the end, Temple tells the Master that Earth never needed to be conquered; all the aliens had to do was ask for help, and it would have been granted. Moved by this revelation, the Master’s eyes well up with tears, realizing a different path to leave Earth unharmed might have been possible.
