The movie opens with a car speeding along a dirt road. The woman driving (Jan Holden) is telling her boyfriend, Chris (Barry Lowe) to "Hold on, well get help soon, were coming to the main road." He has a large dark blister on his face. He's delirious, grabs the steering wheel, and tries to run the car off the road. They enter a "T" intersection and nearly crash into the car Professor Bernard Quatermass (Brian Dunlevy) is driving. Quatermass gets out of his car and walks over to the other. He's very angry and shaken, but inquires about the burn on the man's face. The woman hands him some rock fragments and tells him they were responsible for the burn. She tells him they were in Winterton Flats for a picnic. Quatermass helps push the car back onto the road, when Chris bolts from the car. They wrestle him back into the car. Credits roll. (Note: The British Version is called Quatermass 2, while the American version of this movie was titled Enemy from Space.)
At the Rocket project we see a building with a radar antenna mounted on the roof. Two operators are watching the screen. They notice scores of objects coming down slowly. They track the objects to one hundred miles north of their location. As they reset the equipment, Quatermass drives up. He notices the antenna setting and is very annoyed. He scolds Brand (William Franklyn) and Marsh (Bryan Forbes) for what he perceives as unauthorized use of the scanner.
He tells Marsh and Brand that tomorrow none of them may have jobs. He gives Marsh the stone fragments and tells him to have them analyzed and classified. He apologizes to Brand and tells him that the moon project is to be cancelled. He outlines the project scope in front of a scale model--to build a colony on the moon. As he prepares to get in his car he points to the rocket ship on the launch pad and tells Brand, "Can you still bear to look at it? The sum total of our achievement up to now--one Rocket, crewless, untried." He leaves for the rocket launch area. Brand returns to the radar room and notices more traces on the screen.
It is morning and the office is a buzz of activity. Quatermass enters, awake all night, and Marsh is waiting for him. He wants to report on the meteorite. Brand is holding the reconstructed fragments and notes that it is hollow and symmetric. Quatermass tells Marsh to get the car ready. He and Marsh drive up to Winterton Flats to have a look at the place themselves. They encounter a new road, marked "Road Closed, Government Property." Ignoring the sign they drive until the road ends and they get out of the car to investigate. Quatermass warns Marsh not to walk any further. They get back in the car and drive back and take another fork in the road. That takes them to a section of road on a plateau. They see a demolished town below and their moon project fully built. They walk down to get a better look. They notice the area littered with more rock fragments, some broken, others nearly whole. They hear an alarm horn and Quatermass asks, "I wonder if that's meant for us? Perhaps we better go and explain ourselves." Marsh digs up a whole object. He picks it up and hears a whoosh of air, and smoke billows from the object. Marsh coughs and collapses to the ground. Quatermass notices something on his face. A large blister and a V shaped scar on his cheek. A truck of soldiers leaves the installation. Quatermass helps Marsh to his feet as a dozen or more soldiers surrounds them. A couple of soldiers grab Marsh. They refuse to respond to questions asked by Quatermass. He notices one soldier with the same V shaped mark on his face. They escort Quatermass back to his car after hitting him in the face with a rifle butt. They tell him, "Go, go now." Quatermass drives into a local town to report to the police, but the little town has no police, only a camp committee in the Community Center. He talks to the camp secretary, Dawson (Charles Lloyd Pack). When he asks about the installation he is met with hostility and suspicion. They refuse to allow him to use the phone. Quatermass drives to the nearby town of Brawley. The police there tell him he was not supposed to be trespassing at the installation. They will not tell him anything about the facility. Quatermass drives back to the rocket center. Brand tells him more meteorites have fallen. Quatermass immediately orders the rocket center guarded. He tells Brand the stones are a container of some type. In addition to ammonia gas, they harbor something alien and alive. He also tells Brand that the moon project has already been built at Winterton Flats. Quatermass will go to London in the morning. His staff has reconstructed the container before it entered the atmosphere. They conclude its aerodynamic shape is the sign of intelligence.
Driving to London, Quatermass notices large trucks moving equipment with the same strange logo he saw on the road signs. He stops at Scotland Yard to talk to Inspector Lomax (John Longden). Lomax wants to engage in small talk, but Quatermass gets straight to the point. He asks, "Just how much do you know about a place called Winterton Flats?" Lomax seems stunned. He tells him the place is Top Secret. Quatermass tells Lomax his story. Lomax finally admits a tidbit, "Well that place should be harmless enough. It's a government project to make synthetic food." Lomax then gets on the phone and arranges a meeting for Quatermass with a Member of Parliament, one Vincent Broadhead. They are to meet in an hour.
Quatermass tells Vincent Broadhead (Tom Chatto) about his trip to the site. Broadhead thinks the project is just a waste of money, but he wants details and tells Quatermass it started to scale up about two years before. Broadhead tells him he just got a pass to visit the installation. He meets with the P.R. officer (John Van Eyssen) and gets Quatermass added to the pass. They have lunch then board cars for the trip to Winterton Flats.
They enter the plant. It resembles a huge oil refinery (Shell Haven Refinery in Essex). Quatermass pulls Broadhead aside when he spots the Medical Centre Building. He is looking for his colleague, Marsh, and asks if he wants to join him in having a look inside. The P.R. guide notices Quatermass and Broadhead missing from the tour and searches for them. The two men enter the medical building and are met by a very rude doctor and an empty hospital. There are no patients and very little staff. The guide finds them and gets them back with the inspection team. Quatermass notices a large container marked Ammonia. Their next stop on the tour is one of the large containment domes. Three men inside the dome don gas masks as the inspection party is hustled along the tour. The guide notices that Broadhead is missing. He tries to seal the door on the tour group, but Quatermass escapes and gets back outside. Quatermass runs back to a steel sphere he noticed earlier. Broadhead exits the top of the structure and stumbles down a ladder screaming and coughing. He is covered in a corrosive black slime that burns. He warns Quatermass not to touch him. The material has blinded him and eventually kills him. An alarm horn goes off and Quatermass runs back to the car. Guards with machine guns fire at him. He commandeers one of the cars and escapes the facility.
At Scotland Yard, a very drunk Jimmy Hall (Sid James) stops by for any news he can print in the newspaper. He needs to fill one and one-half columns before midnight. Quatermass stops by to see Lomax. Hall accosts him at the front desk. Lomax is preparing to leave the office. Quatermass briefs Lomax on the goings on at the plant. He tells Lomax that Broadhead is dead. Lomax shows him a newspaper story that Broadhead is away on a "Trade Mission." Quatermass demands an investigation. Lomax goes to talk to the Commissioner (John Stuart) but before he can relate the story he notices the mark on the man's hand. He makes up a story and exits. He returns to Quatermass and asks about the mark. He describes it perfectly. They now know that many government and police officials are being systematically infected to protect the project. Brand calls Quatermass at Scotland Yard and tells him they've found the source of the containers. Jimmy Hall barges into Lomax's office and Quatermass and Lomax decide that a news report may be the best solution.
At the Rocket Center Quatermass and Lomax show Hall the evidence. He tells him that the alien intelligence is a piece of a larger organism. They infect a human host then, "an immediate invasion of his entire nervous system. Something is implanted, an instinct, a blind compulsion to act for them." Hall is incredulous. He wants more information and wants to see Winterton Flats. Before they leave Quatermass briefs Brand on his contingency plans. He also gets a preliminary report on the black slime. He tells Hall and Lomax that it is a deadly poison to every living thing on earth. Quatermass thinks it is food for the life in the domes.
There is a St. Patricks Day dance at the Rec Center. Ernie, the bartender (Michael Ripper) and Sheila (Vera Day) are busy behind the bar with drink orders. Sheila is horrified by the drunken "jig" Paddy Gorman (Percy Herbert) is performing. She pushes him aside, dances, and wins the competition. Hall shows up, along with Quatermass and Lomax, and asks Sheila about work at the plant. The camp secretary recognizes Quatermass and tries to throw the three out. Then the crowd tries to throw them out but Lomax identifies himself as a police officer and tells the crowd they are in danger. A container falls through the roof. They empty the room while Sheila picks the meteorite up and is infected. Against strong advise from Quatermass, Hall calls in his story from the Rec Center. The guards show up and kill Hall while he's on the phone. Quatermass and Lomax get in the car and are fired upon as they depart. They head for the facility. Quatermass hits and kills a guard on the road with his car. They stop and find a truck with steel containers containing alien creatures. He notes that this is how they get them back to the domes. Quatermass sends Lomax back to London, while he impersonates the dead guard to gain entrance to the facility.
A mob of townsfolk walk to the plant. They stop Lomax from getting back to London. They steal his car and head for the gate. Quatermass arrives at one of the dome processing areas. He looks into the dome through an inspection port and is mesmerized at the sight of the alien creature. It is huge. It looks like a multi-eyed, dark writhing carpet. His fellow workers suspect he isn't one of them, but before they can act, a general alarm is called to an emergency at Gate 4. The mob approaches the gate. The mob attacks one of the container retrieval trucks. The guards retaliate with gun fire. They are told over the loudspeaker to shoot to kill. They fight their way into the main pressure control blockhouse, where Quatermass was waiting for them. He identifies himself. Dawson, the camp secretary, panics and wants out. Quatermass examines the controls and notes the gases being used: Ammonia, Methane, Hydrogen and Oxygen. He also examines the piping layout. He speculates that the oxygen is being used to acclimatize the aliens to the earth's atmosphere. He shuts off all gases to the domes except Oxygen. That gets an immediate response. Quatermass confides in Lomax that the contingency plan is to use the rocket ship as an atomic bomb to hit the orbiting asteroid--the source of the containers and the aliens. Launch at midnight is the plan. Brand and Peterson (Edwin Richfield) make preparations at the launch center. As they are setting the systems, Marsh arrives with a contingent of guards to stop the launch. Brand is shot but launches the rocket. With his dying strength he arms the rocket for detonation.
At the blockhouse, the mob has armed themselves with anti-tank weapons. Quatermass is happy to see his rocket was launched. The plant supervisors try to bargain with the control room mob. McLeod (John Rae) talks to one on the phone and arranges to see inside the dome. He, Dawson, and a third man leave and walk to one of the domes. Shortly thereafter there is a scream that reverberates through the oxygen pipe. Then blood starts dripping out of the oxygen supply pipe. They used the men as a plug to stop the flow. Angry, Ernie and Paddy begin firing at the dome and manage to blow one up. A toxic gas wafts towards the control room. The alien creature emerges from the remains of the damaged structure. It writhes around and attacks the control room, pushing in a wall. The remaining group consisting of Quatermass, Lomax, Ernie and Paddy escape. Amid the confusion of the humans, and the destruction to the plant by the alien forms they commandeer a jeep and escape the plant. Some distance from the plant they look back at three creatures writhing in pain exposed to the earth's atmosphere. The rocket hits its target in space and explodes lighting up the night sky like daytime. The creatures fall over and die amid the burning wreckage of the plant. A huge wind gust blows over their jeep. They take shelter behind it. An infected guard they took as proof of the infection now shows no sign of the mark. We end with Lomax asking, "How am I going to make a final report about all this?" Quatermass responds, "What worries me is how final can it be?" They walk down the road, and the cast credits roll.