"How do you kill mud?" asks the hero of this movie, a question that has surely never been asked before or since, except perhaps by a few people who really hated glam rock. This early Hammer film was made before they discovered gothic horror. In 1956, their biggest hit so far had been the sci-fi shocker "The Quatermass Xperiment", and this movie is a blatant attempt to copy that formula right down to the clumsy title including a capital X to prove it's not kids' stuff, as sci-fi tended to be in those days. It's also very similar to "The Blob", and would be an obvious rip-off had it not been made two years earlier, so maybe it inspired Steve McQueen's bizarre screen debut. Another thing it's extremely similar to is early seventies "Doctor Who"; it's easy to imagine Jon Pertwee coping with the strange problem of a monster that eats nuclear power stations (many modern viewers will no doubt be rooting for the monster). But in 1956 Time Lords hadn't been invented yet, so the task falls to a certain Dr. Adam Royston.
Herein lies the main reason why this film isn't the minor classic it could have been. Just as the charmless and permanently drunk American actor Brian Donleavy was cast as the title character in the first two Quatermass movies and ruined them, Dean Jagger plays the hero of a film set in Britain where everybody else is British because Hammer thought this would improve its chances of being a hit in the USA. Which turned out not to be the case, because he's absolutely dreadful. He's got as much charisma as the mud he's trying to kill, and sometimes he pauses in mid-sentence in such an odd way that I'm pretty sure he was reading his lines from cue-cards. Peter Cushing would have carried it off magnificently, especially the laugh-out-loud "scientific" explanation for all this nonsense, which is worthy of Ed Wood, but alas, this is proto-Hammer, and none of the familiar faces are present (except my uncle's old drinking buddy Michael Ripper, who was in just about all of them somewhere or other - that's him playing the army sergeant).
The other problem is the monster. In the first half, where we don't see what it is, only what it does - and one of the deaths is very graphic indeed for 1956 - it's genuinely menacing. And then we find out that it looks like chocolate pudding, and for some reason it's not scary any more. Also, its actions become totally predictable a bit too early, meaning that all the peril in the latter part of the film is extremely contrived. But overall this is nearly a very good sci-fi/horror B-movie, let down by one terrible bit of casting.