On a Scottish moor, soldiers learning to use the Geiger counter discover unexplainable radioactivity in the vicinity. Pvt. Lancing reports that he has seen liquid bubbling out of the ground, but suddenly, before the phenomenon can be checked out, a great explosion occurs, which kills Lancing and injures another soldier. The area is quarantined and an American, Dr. Adam Royston, who is the chief atomic scientist at a Scottish research laboratory at Lochmouth, is called in to advise. By the time Adam arrives, the radioactivity has mysteriously vanished and he is left to ponder a bottomless fissure that has formed on the moor. That night, Willie Harding and Ian Osborn, two mischievous boys acting out a dare, walk through the woods toward an ancient “haunted” tower ruin, where a hermit, Old Tom, resides. Willie is approached by a frightening creature and, later in the night, must be hospitalized for first degree radiation burns, from which he eventually dies without regaining consciousness. The next day, Adam checks out the tower and finds a sample container from his personal workshop. The container, which once held radioactive trinium, is now empty, and when Adam questions Tom about it, he has no idea about how it got there. After returning to work, Adam finds that his laboratory is in disarray and a large amount of trinium he has been working with is missing. Upon receiving the report of the stolen trinium container, the United Kingdom Atomic Energy Commission sends “Mac” McGill from their Internal Security Division to investigate. The director of the research facility, John Elliot, is unhappy that Mac has come and at first tries to block his investigation, but Mac soon forms an easy alliance with Adam. Mac is supportive when Willie’s father accuses Adam and his fellow scientists of “letting off bombs you can’t control.” Adam, sorry for the man’s grief, tells Mac that “we only try to create, not destroy.” Oblivious to recent events, a nurse and an intern enter the hospital’s X-ray or “radiation” room for a tryst. Their lovemaking is interrupted when something seeps in through the vents and attacks the intern, who melts away from radiation burns, leaving the nurse injured and mentally unstable. Seeing the aftermath, Adam suggests to his colleagues that the creature can take different shapes to conform to its environment, like oil. After learning that two more soldiers were killed while guarding the moor, Adam and Mac investigate. Later, Adam theorizes that intelligent life that requires radiation to exist has developed under the earth’s crust, and over time its living space has been compressed. To survive, Adam suggests, the life-form surfaces, but only in the last few years, due to the development of nuclear power, has there been enough radiation above ground to sustain its life. Extrapolating his theory, Adam further suggests that the creature will grow and become more dangerous as it consumes more energy and that the research facility, which houses a cobalt bomb, will soon attract it. Elliot calls Adam’s theory “rubbish,” but Maj. Cartwright, who is the army liaison, Mac, and Elliot’s son Peter take Adam seriously. When Adam determines that the fissure will have to be investigated, Peter, who, against Elliot’s wishes, wants to be a researcher rather than an administrator, volunteers to be lowered into it. On his way down, he passes a human corpse, then abruptly signals to be raised and barely misses being attacked by the creature from down below. The major is ordered to have his men concrete over the fissure, but both Mac and Adam, who refers to the creature as “this X, this unknown quantity,” believe that the creature will break through and attack again. Later, their fears are confirmed when the creature is reported to have “melted” four people in a car and Adam, by looking at a map, determines that it is moving on a straight course toward the research facility. Although Adam attempts to move the cobalt bomb, the oozing, radioactive, mud-like creature soon attacks the guard at the front gate, forcing Adam to order an evacuation. To give the creature a clear path for its return, the inhabitants of the area take shelter in a chapel, while Adam and Peter work furiously to devise a way to destabilize the creature. Although their laboratory experiment is only partially successful, the urgency of the situation compels Adam to try it out in real life. From helicopters, soldiers see that the creature has changed course, but static in the atmosphere, due to the creature’s radioactivity, prevents them from reporting to the ground. Adam, Elliot, Mac and Peter return to the fissure, assisted by soldiers. Peter lures the creature forward with a jeep filled with radioactive bait. The jeep’s tires stick in the mud, but Peter breaks free and lures the creature into an area rigged with special electronic equipment devised by Adam. When Adam orders the switch turned on, an explosion occurs and afterward the creature seems to have been destroyed. A second, unexpected explosion occurs, which mystifies Adam, but Mac and Elliot agree that Adam’s plan has worked.