A rocket crash-lands in England after a flight of more than 57 hours into deep space.
The design of Professor Bernard Quatermass (Brian Donlevy), a forceful, misanthropic American scientist, the Q-1 had three astronauts aboard when it left Earth, but only one of them, engineer Victor Caroon (Richard Wordsworth), is on board upon landing, and he is in a near-comatose state.
Even more baffling, the spacesuits of the other two men are still aboard the wrecked ship and are still interlocked, as though they were in them when whatever transpired.
Quatermass's investigation is complicated by the presence of Inspector Lomax (Jack Warner) of Scotland Yard, who is treating the disappearance of the two men as a potential murder case, and by Caroon's wife Judith (Margia Dean), who blames the scientist for what has happened to her husband.
An on-board camera, although damaged, shows an encounter with some form of energy that invaded the ship and attacked the crew, seemingly killing the other two astronauts and rendering Caroon unconscious.
Caroon's condition keeps worsening -- Quatermass's medical expert, Dr. Gordon Briscoe (David King-Wood), is alarmed by the man's impossible heart- and pulse-rate, his degenerating skin and apparent changes in his bone and facial structure.
Judith Caroon tries to spirit her husband out of the hospital where he's being cared for, not knowing that something horrific is happening to him. Quatermass and Briscoe soon realize that Caroon is little more than the shell of a man, masking an invading alien life form that can literally draw the life out of any living thing that it touches.
The manhunt turns into a fight for survival as the creature continues to kill and mutate, threatening to release spores into the air and spread itself by the millions throughout the Earth.