Appearing as an energy form over a London driven mad by primal and evolutionary triggers dating from the beginning of the Human species, it was not truly a living being, but rather the memory-remnant focus of Martians of times past, who had placed traces of their own DNA into primitive Earth Hominids. It was their hope to alter these creatures enough to be used as their new bodies, but the genocidal/suicidal war on Mars ended their race's existence before this could come to fruition.
As work began on a new subway line in a part of London called Hobbs' End, excavation uncovered what was initially believed to be an unexploded German missile from late in World War Two. Consulted for his expertise, scientist and reluctant adventurer Doctor Bernard Quatermass discovered that the metal cylinder was in fact a spacecraft, buried for millions of years, determined to be of ancient Martian origin, and containing the corpses of its crew, insectoid beings far more advanced than our kind, even in the present. But while Quatermass clashed over the craft's disposition with the same forces in the British military that had turned his rocket program from one of civilian space exploration to orbital weaponry, odd things began to occur.
Hauntings finally drove the last residents out of Hobbs' End, something construction crews and bureaucrats had been unable to do. It turned out that Hobbs' End was once called Hob's End, Hob being short for Hobgoblin and indicating legends of demonic presences going back many hundreds of years. Quatermass came to the startling and alarming conclusion that perhaps all hauntings and such throughout Human history were in fact the work of a recessive gene in Humans, a dormant telepathic and telekinetic talent left over from Martian experimentation on our forebears. Using a device to access memories on a nearly racial level, he viewed a conflict on Mars driven by minor differences that caused Martians to fight each other nearly unto extinction. Again opposed by his enemies within the rocket program, Quatermass was hard pressed to prepare as the events cycled up from mere poltergeist-type activity to true racial warfare.
Humans who still had the recessive Martian genes joined their powers in a collective mind and began mass killings of Humans who did not have this, as well as animals and other species. Both Quatermass and his personal assistant fell victim to this genetic trigger, until the ambient energies now unleashed enough psychic residue for a Martian consciousness to gather over London's skies, an energy form that looked like the glowing head of the ancient extinct Martians. It looked very much like a Horned Devil, and its mere presence accelerated the killings and violence, even spreading between Humans who had the gene against one another.
Doctor Matthew Roney, an unaffected associate of Professor Quatermass plotted with him in his lucid moments to try and combat this entity. They had earlier reasoned that Martian programming had lead to racial memories of a great horned enemy of mankind, the inspiration for the Devils of myth and religion. Now, they recalled that many other legends that had demons and magical beings vulnerable to iron and water. Realizing that iron could disrupt the mental energies being used in the slaughter, Roney sacrificed his life linking a metal chain from the Martian Craft to some wet earth. Since the apparition had become the focal point of the disturbance, destroying the craft made it dissipate and people around the world return to normal. The Horned Martian Devil still in his thoughts, Quatermass warned against letting petty differences do to Earth what race war did to Mars.
The ending of the feature film differed only slightly. In this, Roney ascended to a large construction crane, which he swung directly into the energy creature, whose iron disrupted and destroyed it, still costing Roney his life, this while Quatermass prevented his possessed personal assistant from stopping Roney. The entity and the crane created a spectacular explosion, probably owing to the dramatic needs of a feature film.