On a desolate Scottish moor, a platoon of soldiers are being trained in the use of the Geiger counter. Their manoeuvres are interrupted by a mysterious and unexplained explosion which leaves a large fissure in the ground.
Dr Adam Royston (Dean Jagger), chief scientist at the nearby atomic research establishment, is called in to investigate and discovers that two soldiers who were nearest to the explosion are suffering from first-degree radiation burns.
Assisted by a band of technicians from the research station, Adam and Peter Elliott (William Lucas), a young administrative worker bent on becoming a scientist, examine the fissure to find that it appears bottomless.
Baffled and unable to discover the source of the radiation, Adam and his colleagues return to the establishment. That evening, two young boys playing near a derelict tower on the moor are confronted by an unknown phenomenon.
Willie (Michael Brook), one of the youngsters, is later found to be suffering from severe radiation burns.
Inspector McGill (Leo McKern), of a special police branch, arrives to assist Adam with the investigations shortly before the ‘unknown’ strikes again, this time in the radiation room of a hospital where it kills a medical orderly, severely injures Zena (Marianne Brauns), his girlfriend, and plunders the radiation store.
Together, Adam and McGill examine the spot and Adam realises how the ‘unknown’ is able to pass, unhindered, into locked rooms.
He believes that it relies on radioactivity for its existence and decides to have the interior of the fissure investigated.
Peter Elliott descends into the chasm and is almost trapped by its terrible denizen.
Knowing that in its quest for radiation the ‘unknown’ will make for the research station next, Adam tries to take the establishment’s cobalt phial to safety. The ‘unknown’ arrives sooner than anticipated, however, and absorbs the radioactivity of the cobalt before returning to the fissure.
With the ‘unknown’ now more dangerous than ever, Adam and McGill agree that it must be destroyed immediately.
Accompanied by a party of technicians, they assemble at the fissure where the evil occupant is enticed from its lair by radioactive ‘bait’. The final engagement between man and his mysterious enemy is underway . . .
One of a series of sci-fi films produced by Hammer in the 1950s, X the Unknown was initially set to be directed by blacklisted Hollywood émigré Joseph Losey before illness struck, leading to his replacement Leslie Norman.
The script was provided by one of the studio’s better-known writers, Jimmy Sangster, who went on to write some of Hammer’s classic horrors.
Dr Adam Royston