In 1964, astrophysicists on earth become aware of an immense cloud of gas that enters the solar system. The cloud, moving to interpose itself between the sun and the earth, could wipe out most of the life on earth by blocking solar radiation and ending photosynthesis. A cadre of astronomers and other scientists is drawn together in Nortonstowe, England, to study the cloud and report to the British government about the consequences of its presence.
As the behaviour of the cloud proves to be impossible to predict scientifically, they come to the conclusion that it might be a life-form with a degree of intelligence. In an act of desperation, the scientists try to communicate with the cloud, and to their surprise succeed in doing so. The cloud is revealed to be a super-organism, many times more intelligent than humans, and who in return is surprised to find intelligent life-forms on a solid planet.
When the astronomers ask the cloud how its lifeform originated, it replies that they always existed. One of the characters suggests this is incompatible with the Big Bang theory. Thus it may be that Hoyle was hinting at his own Steady State theory of the existence of the universe, which has since been disproved by the discovery of cosmic background radiation.
The cloud then learns that another intelligent cloud has stopped communicating and may have mysteriously vanished. This happens around another star, not too distant in the cloud's terms. So the cloud decides unexpectedly to move on. Two of the scientists die in an attempt to learn the cloud's own language through visual signals, in order to gain further insights about the universe.