At an atomic energy research establishment in a remote area of Scotland, Dr Adam Royston, an independent-minded scientist, is taken to task by the establishment's manager, John Elliott, for spending too much time on his own experiments at the expense of his government-funded work.

Ordered to investigate an incident that has taken place locally during an army exercise, in which an explosion occurred following the detection of radiation, Royston discovers an apparently bottomless fissure on the site of the blast.

The following morning, Royston examines a young boy admitted to a local hospital with radiation burns (from which he soon dies) after encountering something in the woods the previous evening. Returning to the research establishment, Royston discovers that his private laboratory has been broken into, with a lead container, in which he had housed the radioactive material trinium, having been melted by a form of extreme heat. There is now no trace of radiation.

A government investigator, McGill, is called in following the apparent breach of security, and he joins Royston in the investigation of another incident, this time in the radiation room of the hospital, where a staff member has been liquefied and a safe, which had contained radium, melted. As with Royston's laboratory, there are no signs of forced entry, although Royston notes that whatever entered the room must have done so via a metal grille in the wall.

Royston advances the theory that energy, trapped at the centre of the Earth during the formation of the outer crust, may, over millennia, have developed corporeal form and intelligence, and is now attempting to reach the surface. When this had been achieved in the past the energy had disintegrated because there had been nothing for it to feed on. But now, thanks to the abundance of radioactive materials, the energy that has emerged from the fissure can continue to grow as long as it continues to find sufficient nourishment.

Although this theory is greeted with a degree of scepticism by the others, John's son, Peter, volunteers to go down the fissure in an attempt to locate the presence of any such 'energy'. Lowered on a bosun's chair, Peter first finds the partly liquefied body of a soldier who had been reported missing while guarding the site, before seeing something fast approaching him from the fissure's depths. He is quickly hoisted to safety.

Now convinced that some sort of creature is actually present, the Army burns out the fissure and cements it over in the hope of containing whatever resides within. But later that evening, the creature, a large, tar-like, glutinous mass, forces its way through the cement barrier and heads directly for the research establishment, where it consumes a large quantity of radioactive cobalt stored there.

Speculating that the bigger the creature becomes the greater the range over which it will be able to detect radioactive materials, Royston calculates that its next target will be a nearby experimental nuclear power station. It is decided that the best place to attempt to destroy the creature is the fissure, where it is expected to return before reemerging to feed.

Royston explains how his experimental work on developing the means to incapacitate atomic weapons (by passing radioactive materials between scanners calibrated to a certain pitch) may be the only means by which to destroy a creature made of pure energy. Although Royston's experiments remain inconclusive, McGill convinces him to make the attempt.

After two prototype scanners have been mounted on either side of the fissure, the creature is lured to the surface by a canister of cobalt affixed to the rear of a jeep. As the creature emerges the scanners are turned on. The creature stops and begins to glow, before bursting into flames and disintegrating.

As the party of relieved onlookers approaches the dwindling flames, an explosion occurs within the fissure. Royston, puzzled by this occurrence, stands looking into the fissure, but the others depart, convinced that the creature has been destroyed.