Giant mud monster terrorizes Scottish countryside, hungry for radioactive materials.
That one sentence synopsis may not sound like much, but this is the greatest British science-fiction movies of the Fifties. The monster has no motivation or purpose other than to feed, which is what makes it truly frightening, much like The Blob, which this film predates and akmost certainly influenced.
The various characters are all well-realized and believable, especially McKern's affable atomic inspector. Jagger is the least obnoxious of the many American actors British science-fiction of this time felt necessary to shoehorn into their productions. Chapman's toad-like bureaucrat is tiresome, but realistic, much moreso than the noisy twits portrayed by Peter Copley and Edwin Richfield in Quatermass and the Pit, the greatest British sci-fi film of the Sixties. Chapman at least admits his mistakes. Interesting to note that 'X the Unknown' comes out of a Y-shaped fissure; perhaps a subtle hint that the biggest monster is what comes from the X-Y chromosome?
Perhaps most importantly, the film is not afraid to be nasty when it needs to be. How many other films of this time (or any other) have allowed children to die of radiation poisoning? Certainly Neil Hallett's grisly death scene gave me nightmares as a kid, and is still quite grisly today. Incidentally, that is probably one of the earliest horror scenes that equates sex with untimely death, something slasher films later made de rigueur.
Note: Whatever IMDb says, it's highly doubtful Archie Duncan (and possibly some of the others at the bottom of this page) are in this film.