On a quarry by the Scottish village of Lochside, soldiers undergo a radiation detection exercise. Latecomer Lancing (Kenneth Cope) finds an unexpectedly strong radiation emission. With a small explosion, the earth above it splits apart in an indeterminately deep fissure, giving Lancing radiation burns. From a nearby radiation lab, Dr. Adam Royston (Dean Jagger) is called to investigate.
That night, local boy Willie Harding (Michael Brook), on a dare, meets something in the woods which gives him similar burns to Lancing. Royston then finds his workshop to have been raided, with a trinium sample drained of radioactivity. Inspector McGill (Leo McKern) learns that several people have died in bizarre circumstances — in which their bodies appear to have melted. So begins the search for "X", an assailant which can cross barred windows, absorb radiation, and leaves traces of radioactive mud...
This film provides examples of:
- Agent Scully: Administrator John Elliot, impatient with Dr. Royston's ambitious experiments, staunchly doubts the emergence of a subterranean predator.
- The Apprentice: Young Peter Elliot aims to follow the work of Dr. Royston. His note of a possible cause of test failure persuades Dr Royston to try field use of the radiation scanners.
- Attack of the 50-Foot Whatever: X grows big enough to cover fields and fill village streets.
- Blob Monster: A living blob of amorphous earthen matter, X, descendant of a species which evolved on a radioactive Earth, emerges from miles beneath the ground in search of a recent abundance of radiation. It can slip through window bars and ventilation grills and grows to colossal size.
- Body Horror: When an enlarged X seeps through a ventilation shaft into the hospital, contact with its radioactive body melts off the bone the flesh of Dr. Kelly.
- The Brigadier:
- Major Cartwright is readily cooperative with Dr. Royston and co.
- In a police variant, Inspector McGill supports Dr Royston's evaluation of the bizarre threat.
- Captain Ersatz: After the success of The Quatermass Xperiment, Hammer immediately wanted to make a sequel. However, they couldn't get the rights however, so they made X the Unknown, giving it an extremely similar plot-structure and atmosphere to the first and a highly Quatermass-like hero in Dr. Royston.
- Death of a Child: Willie eventually dies of radiation burns.
- Death by Sex: In the hospital radiation room, a secret liaison with Nurse Zena leads Dr. Kelly to a graphically fatal encounter with X.
- Don't Go in the Woods: Young Ian Osbourne and Willie Harding, on a mutual dare in the local woods, encounter X.
- Family-Unfriendly Death: Proximity to X boils away the facial flesh of Dr. Kelly, revealing the skull beneath.
- Freud Was Right: In Hammer and Beyond: The British Horror Film, Peter Hutchings notes parallel of a monster released by a crack in the earth with, in the recent The Quatermass Xperiment, a mutation spread by a crashed rocket.
- Friend to All Children: Dr. Royston gently coaxes information from traumatized young Ian Osborn.
- Eagleland: Averted with Dr. Royston.
- Good Shepherd: The village reverend conscientiously steers villagers to the relative safety of the church and pulls a small girl to safety when X approaches.
- Kid Amid the Chaos: An infant girl, innocently oblivious to the danger, nears a crumbling churchyard wall, behind which approaches the colossal radioactive predator, and is narrowly pulled to safety by the reverend.
- Nothing Is Scarier: Little is seen of the mysterious attacks until later on.
- Nuclear Nasty: Dr. Royston deduces X to be of a species which, as Earth grew less radioactive, withdrew to the core. Roused by a cyclical gravitational tilt, it emerges in search of the recent abundance of radiation.
- Scenery Porn: Some lovely shots of the village and surrounding landscape.
- Science Hero: Dr. Adam Royston, in an attempt to neutralize radioactivity via two rotating scanners, manages to direct the principal against X.
- Screaming Woman: Played for Horror with Nurse Zena when Dr. Kelly is horrifically killed.
- Stripped to the Bone: Exposure to the rapidly growing X causes human flesh to melt off the bones.