When Hammer found international success with The Quatermass Xperiment they knew they quickly wanted to repeat their success and created a new sci-fi horror story but found that Quatermass creator Nigel Kneale refused to allow them the rights to Quatermass. Kneale didn’t want his characters used in a story not written by him, so after quick renaming of the main character, Hammer was ready to make X the Unknown.
Blacklisted director Joseph Losey was set to direct by star Dean Jagger refused to work with him so Losey removed and Leslie Norman was brought in at the last moment.
A group of soldiers are on maneuvers in rural Scotland when they discover an explosion that causes a large crack in the Earth and that kills a couple of the soldiers. Dr. Adam Royston (Dean Jagger) is brought in to investigate. The crack is emitting a large amount of radiation. Soon various soldiers and townspeople are turning up dead from radiation burns. Something has escaped from the depths of the Earth and it needs radiation to survive.
The movie is a slow build in the tension, holding off to showcase the monster until near the end of the film when what looks like the Blob’s predecessor makes an appearance. Jagger is essentially playing a variation of Quatermass but does a solid job. The movie was another hit for Hammer as the studio continued it’s path to eventually being known as the masters of horror.