Astrophysicists in 1964 discern an immense cloud of gas entering the solar system. The cloud, moving to interpose itself between Sol & Terra, could wipe out most life on earth by blocking solar radiation & ending photosynthesis. A cadre of astronomers & other scientists is drawn together in Nortonstowe, England, to study the cloud & report to the British government about the consequences of its presence. As the behavior of the cloud proves to be impossible to predict, they conclude it might be a life-form with a degree of intelligence. Desperately, the scientists try to communicate with the cloud--to their surprise, successfully. The cloud is revealed to be a super-intelligent organism, who is itself surprised to find intelligent life-forms on a solid planet.