Royston begins his investigation after a seemingly bottomless chasm opens in an empty field where British soldiers are conducting practice sessions. Manned with Geiger counters, they search for hidden canisters of "harmless radioactive material." But eventually one of the soldiers sees his Geiger counter go haywire and with a blast of light and a rumble like thunder, the ground opens ups. The soldier suffers severe radiation burns. Initially, Royston is baffled, but soon additional events attract his attention: two children experience radiation burns while playing in the woods and Royston's own laboratory is mysteriously vandalized.

Soon Royston realizes he's up against an elder force of near Lovecraftian implications. Jimmy Sangster's screenplay treats this scenario with complete seriousness. You won't find any of the camp attitude prevalent in The Blob. Sangster focuses on the story's more horrific and suspenseful elements--as when soldiers maintain a lonely vigil at the chasm. And the story takes occasional forays into laboratory explanations. But precious little time is given to developing the characters. However, director Leslie Norma has such a high caliber group of actors to work with, including the great Australian character actor Leo McKern as a police inspector, that the characters always have depth and interest. In a supporting role, Anthony Newley plays a soldier who must serve as sentry near the chasm. and while he's on screen, the movie slows down to let us get to know his character. But the rest of the movie remains very focused on plot and science.

X The Unknown is filled with effectively eerie sequences, but the best sequence comes when Royston and the soldiers rig up a winch over the chasm and lower a team member into the depths to search for traces of radioactivity. (It's like a trip into a cellar during a horror movie.) This is one of Hammer's best forays into science fiction. It's marred by some ineffective model work that significantly lessens the impact of the final sequence (however, close-up shots of a bubbling mass are frequently quite eerie).

Now available on DVD from Anchor Bay Entertainment, X The Unknown is required viewing for all sci-fans. The DVD also contains a so-so episode of the TV series World of Hammer titled "SCI-FI" that supplies some background on Hammer's science-fiction films (although this extra has already appeared on other "Hammer Collection" releases from Anchor Bay).