Workers digging an extension to the London Underground at Hobbs End uncover a skull and a nearby fragment of a metallic object. Palaeontologist Dr. Mathew Roney, James Donald, identifies the skull as a five-million-year-old apeman, older than anything previously found. The metallic fragment suggests something far more mysterious than a wartime relic, and the team calls in an army bomb disposal unit to assess the danger.
Meanwhile, Professor Bernard Quatermass, Andrew Keir learns that his Moon-colonisation plans are being taken over by the military, who want to establish missile bases in space. Colonel Breen, Julian Glover is assigned to Quatermass's British Experimental Rocket Group, and when the bomb disposal team requests his assistance, Quatermass accompanies him to the site. A second skull is found inside a chamber of the “bomb,” making it clear that the object itself is also five million years old. Noting the craft’s heat resistance, Quatermass suspects an alien origin.
Barbra Judd, Barbara Shelley, Roney’s assistant, accompanies Quatermass to the site. She becomes intrigued by the area’s name, recalling that “Hob” is an old name for the Devil. A policeman mentions a legend about a haunted house opposite the station, and the three of them head there to investigate. Quatermass and Barbara uncover historical accounts of hauntings and other spectral appearances, all of which seem to align with disturbances in the ground around Hobbs End.
A bomb-disposal team member witnesses a spectral apparition of the apeman materializing through the object’s wall. An attempt to drill open a sealed chamber with a Borazon drill fails, but a small hole eventually appears. The hole widens to reveal the corpses of insectoid creatures with horned heads, suspected to be from Mars. Their physiology hints at Martian origins, and they resemble the Devilish imagery tied to the area; Quatermass concludes the spaceship is the source of these visions and disruptions.
They reveal their findings to the press. Quatermass theorises that the ship’s occupants came from a dying Mars and, unable to survive on Earth, sought to preserve part of their race by guiding the intelligence of and imbuing Martian faculties into the indigenous primitive hominids. In Quatermass’s view, the insectoids used medical and surgical techniques far more advanced than those of early Earth. Breen is skeptical, suggesting the “alien craft” is Nazi propaganda intended to sow fear, while a government minister contemplates unveiling the ship to the public.
The drill operator, Sladden, Duncan Lamont, is later overcome by a psychic force and flees, his mind unleashing telekinetic surges that disrupt people and property. He ends up in a church, and before recovering, experiences a vision of insect creatures beneath an alien sky. He sees himself as one of them and feels compelled to escape to save his life.
At Hobbs End, Quatermass brings a machine that taps into the primeval psyche. As he tries to replicate the conditions that affected Sladden, he notices that Barbara has fallen under the spaceship’s influence. Using the machine, he records her thoughts and presents the recording to the minister and other officials. The footage appears to show Martians engaging in a genocidal purge to cleanse Martian hives of mutations, but the minister and Breen dismiss the recording.
A power line is dropped into the craft, jolting it and amplifying the spaceship’s influence over Londoners. The populace spirals into violent, diffuse rage, attacking anyone perceived as different, with deadly telekinetic displays of energy. Breen is drawn toward the ship and is killed by the emanating energy. Quatermass also falls under the alien sway but is snatched back by Roney, who remains unaffected. A portion of the population proves immune, while the psychic energy tears at streets and buildings. A spectral Martian image, evoking the Devil, towers over London.
Recalling legends that the Devil could be defeated with iron and water, Roney theorises that the Martian energy can be discharged into the earth. He climbs a building crane and swings it into the image; the crane bursts into flames as it channels the energy away, killing Roney. With the energy dispersed, the spectral image and the disturbances vanish from the city.
