In a bleak near-future of gangs ruling the streets, Professor Quatermass is accosted by thugs while on the way to a television studio to offer commentary and analysis for Britain's part in an international satellite broadcast covering a US-Soviet space link-up (similar to the Apollo-Soyuz mission in 1975). He is rescued by a young space scientist, Joe Kapp (Simon MacCorkindale), who is on his way to appear in the same programme. During the transmission, Quatermass is harshly critical of the space mission, and predicts that missions like it are wasteful and probably doomed in some way because they don't produce anything useful. Not long afterward, malfunctions begin to show up, and the mission fails, with all lives lost, due to unknown causes. Quatermass and Kapp are forced to flee to the latter's remote lab, which is near a stone circle called the Ringstone. A group of young religious zealots marches to the circle, apparently to take it over for some reason of their own...

Following the devastation at Ringstone Round, Quatermass hears of similar incidents worldwide and returns to London with district commissioner Annie Morgan and a Ringstone 'survivor' Isabel. En route they become separated in a shootout between rival gangs. Meanwhile, the beam of light descends again - near Joe Kapp's home.

Quatermass is rescued by a group of elderly people living in a scrap yard. At the hospital, the doctors are shocked when Isabel levitates off her bed and explodes in a cloud of dust. Elsewhere, the devastated Kapp is left alone in the ruins of his cottage and observatory. More and more young people are joining the Planet People, including the gangs that have been terrorising the cities and the soldiers assigned to keep them away from the Megalithic sites. Contact is restored with Chuck Marshall and with the Russians in the form of Gurov (Brewster Mason). Quatermass theorises that this is not the first time this has happened; Megalithic sites such as prehistoric stone circles are in fact markers where beacons have been left by the alien force for its next visit. Quatermass believes the force is a sphere of energy surrounding the Earth. The Russians and the Americans send a space shuttle, commanded by Marshall, to make contact with the force. Quatermass is sceptical; he believes they are dealing not with an intelligence but with a machine constructed to harvest human protein. The space shuttle is destroyed but not before Marshall reports a giant beam of light stretching in all directions. Meanwhile, the Planet People are gathering at Wembley Stadium in the tens of thousands. Annie and Quatermass travel to the stadium but are powerless to prevent the Planet People from gathering. When they are attacked, Annie drives their car into the underground car park beneath the stadium where she crashes the vehicle and is killed. The lightning strikes the stadium.

Quatermass emerges from the car park to find the stadium empty. So many have now been harvested that the particles of dust in the air have turned the sky green. Kapp attempts to repair his equipment in order to contact the alien presence, but the Planet People destroy his efforts. They try to persuade him to join them but he refuses, convinced now that his family are dead. Quatermass, aided by Gurov, who has travelled to London from Moscow, assembles a team of scientists to find a solution. He deliberately selects old people for the task as they are immune to the effects of the alien force. Quatermass decides to set a trap. He plans to fake the presence of a large gathering of Planet People at Kapp's observatory and, when the force comes, to detonate a nuclear weapon. Quatermass does not believe this will be sufficient to destroy the alien machine but he hopes that it will damage it enough to make it go away. Kapp volunteers to stay behind with Quatermass to help detonate the bomb. The pair set up the trap and wait. Suddenly, Kickalong appears with a group of Planet People, including Quatermass' granddaughter, Hettie. Kapp tries to warn them away but is shot by Kickalong. The light appears, indicating that the alien force has arrived, but the shock of seeing his granddaughter among the Planet People causes Quatermass to suffer a heart attack. However, aided by Hettie, he is able to detonate the bomb. Later, according to Gurov, "The message was taken. It has not come again. We pray it will never come again".