A British experimental rocket orbits the Earth. After being out of contact for 57 hours, the capsule crashes back to Earth in Wimbledon, London. When the capsule is opened, only one member of the three-man crew emerges, Victor Carroon.

Victor Carroon is subjected to various tests which indicate that he has taken on the identities of the other two men. Quatermass and Paterson find a gelatinous substance spread around the capsule's interior.

Carron is taken back to the crash site and a recording of what happened during the orbit is played back to him, with disturbing results. A group of journalists try to kidnap him, but he, in turn, attacks one of them as he begins to change.

Carroon takes refuge inside a cinema (where a spoof film 'Planet of the Dragons', is on screen). He then goes to a chemist where he appears to in pain and his hand is seen to be badly mutated.

Quatermass picks up Carroon's trail and believes that the mixture of chemicals he has taken will accelerate his metamorphosis. Meanwhile, the killing starts - and Westminster Abbey a pulsating mass is revealed clinging to the building.

A state of emergency is declared and the army is brought in. Quatermass plays the creature the rocket tapes and appeals to the last vestiges of humanity left in Carroon to fight what he has become. The creature finally dies, just before it is due to spore.