Kalgash is located in a stellar system containing six suns: Onos, Dovim, Trey, Patru, Tano, and Sitha in the novel), which keep the whole planet continuously illuminated; total darkness is unknown, and as a result so are all the stars outside the planet's stellar system.
A group of scientists from Saro University begin to make a series of related discoveries: Sheerin 501, a psychologist, researches the effects of prolonged exposure to darkness; Siferra 89, an archaeologist, finds evidence of multiple cyclical collapses of civilization which have occurred regularly about every 2000 years, and Beenay 25 is an astronomer who has discovered irregularities in the orbit of Kalgash around its primary sun Onos. Beenay takes his findings to his superior at the university, Aton, who formulated the Theory of Universal Gravitation. This prompts the astronomers at Saro University to seek the cause of this anomaly. Eventually they discover that the only possible cause of the deviation is an astronomical body that orbits Kalgash.
Beenay, through his friend Theremon 762, a reporter, has learned some of the beliefs of the group known as the Apostles of Flame. They believe the world would be destroyed in a darkness with the appearance of stars that unleash a torrent of fire. Beenay combines what he has learned about the repetitive collapses at the archaeological site, and the new theory of potential eclipses; he concludes that once every 2049 years the one sun visible is eclipsed, resulting in a brief "night". His theory is that this "night" was so horrifying to the people who experienced it that they desperately sought out any light source to try to drive it away, particularly, by frantically starting fires which burned down and destroyed their successive civilizations.
Since the current population of Kalgash has never experienced general darkness, the scientists conclude that the darkness would traumatize the people and that they would need to prepare for it. When nightfall occurs, however, the scientists (who have prepared themselves for darkness) and the rest of the planet are most surprised by the sight of hitherto invisible stars outside the six-star system filling the sky. Unfortunately, because the inhabitants of Kalgash never saw other stars in the sky, their civilization had come to believe that their six-star system contained the entirety of the universe. In one horrifying instant, anyone gazing at the night sky – the first night sky which they have ever known – is suddenly faced with the reality that the universe contains many millions upon billions of stars: the awesome, horrifying realization of just how vast the universe truly is drives them insane.