Britain's first rocket has been successfully launched and returns to Earth but communication has been severed. Only one of the three spacemen (Wordsworth) remains on board whose physical and mental health has been severely damaged. He is put under strict observation but his wife smuggles him out of the hospital only to unleash the biggest threat ever to civilisation. For the survivor is being consumed by an alien force, who gains strength from human flesh to multiple and grow, threatening to engulf and kill all living organisms. Professor Quatermass (Donlevy) must find a way to overcome the thing before it becomes impossible.