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.