The novel is in three parts, which a third-person omniscient narrator relates to the reader.
Earth and the Overlords
Both the United States and Soviet Union are mounting expeditions to study the reaches of space. Their endeavors are interrupted when massive spaceships enter the atmosphere, covering Earth’s major cities. These are the Overlords, who communicate through their leader, Karellen. Karellen only speaks with Rikki Stormgren, the United Nations Secretary-General. The Overlords announce they are taking over international affairs and are ending war and violence. The human race pushes back a few times but gradually settles into peace. The Overlords do conduct surveillance on humans, but are not punitive, even when there is some resistance. Resistance particularly comes from the Freedom League, suspicious that they cannot see the Overlords face-to-face and do not understand their long-term goals, no matter how benevolent they seem. Stormgren and Karellen develop a positive relationship. At one point Stormgren is kidnapped by a radical fringe of the Freedom League. Karellen rescues him, but, after realizing he was a pawn in Karellen’s game to get access to these leaders of the Resistance in order to shut down their disruptive activities, Stormgren decides he wants to at least see what Karellen looks like. He consults a scientist friend who gives him a bright light to illuminate the room and screen where Karellen’s voice comes through. Stormgren gets his look, but Karellen tells him that the Overlords will not reveal themselves to the human race until fifty years from now, which is when humans will be able to handle their appearance.
The Golden Age
It is fifty years later and the Earth is a utopia. There is no war or conflict, religion has largely faded away, people do not have to work too much and get to enjoy their intellectual pursuits and leisure activities. The Overlords reveal themselves, appearing almost like the devils of the Christian faith. It is slightly disturbing to humans but they do not have too much trouble coming to terms with them. The Overlords do not spy on humans anymore but are interested in them as objects of study; in particular, they study the paranormal and psychic research.
There is a party at the house of Rupert Boyce, a “supervet” who has dealings with the Overlords in his profession. At the party are George Greggson and Jean Morrell, a young couple. Jean and Rupert lead an Ouija Board session at the end of the party. Also present are Jan Rodricks, the brother of Maia, Rupert’s wife, and Rashaverak, an Overlord who reads books on telepathy and the paranormal in Rupert’s library. During this session Jan spontaneously asks where the location of the Overlords’ planet is, and receives a star-catalog number. He decides to stowaway on a supply ship, even though he knows that if he is sent back, eighty Earth years will have elapsed and he will not see his family and friends anymore. He takes a drug to put himself to sleep for most of the journey, which lasts only a few weeks.
The Last Generation
Jean and George are now married and have two children: Jeffrey, who is seven, and Jennifer Anne, who is an infant. They have moved to the artistic colony of New Athens, located on an island. The colony was founded to try and produce great works of literature, art, philosophy, engineering, etc., as some of these humans believe the Overlords have rendered the human race creatively stagnant.
One day a tsunami hits and Jeffrey is almost killed, save for the intervention of a mysterious voice helping him run from danger; George privately believes this is an Overlord. It becomes clear that an Overlord did intervene to help Jeffrey, as he is the first child to exhibit what all children under ten will begin to exhibit –their evolutionary progression. All children begin to show signs.
Karellen speaks to the world and tells the human race that it is dying out; the children, which the Overlords were sent to observe and prepare for, will evolve and join the Overmind, the overarching cosmic force comprised of ancient civilizations and alien races. The parents of these children cannot join them, and the children are to be taken away. The community of New Athens commits mass suicide by a nuclear bomb after the children are removed to continue their evolution with the Overlords’ supervision.
The Overlords cannot join the Overmind but are his middlemen in terms of preparing the species for their transformation. The children reside on a continent of their own.
Jan Rodricks comes out of hibernation and is told he will be returned to Earth. While in space with the Overlords he is able to observe their cities and behavior. He catches a glimpse of the Overmind, which appears as an eye. When he comes back to Earth he sees only the Children, and is shocked by how they are fusing into one single entity and are no longer very human at all.
It becomes clear that it is no longer safe for the Overlords to watch over the Children and they prepare to depart. Jan decides to stay and chronicle what happens. The Children destroy Earth; Jan, who first feels a sense of emptiness when the Overlords leave and then a profound sense of fulfillment, dies as he is narrating this destruction.
Karellen says goodbye to the human species. It is alluded to that he has a larger plan to try and overcome the inability of the Overlords to evolve and join the Overmind.