(From Wikipedia)
The humans' arms race is brought to a halt by the sudden appearance of mysterious spaceships above all the Earth's great cities. After a week of silence and increasing tension, the aliens, who become known as the Overlords, announce by world-wide broadcast that they have benign intentions and desire to help humanity. As enforcers of peace, they bring salvation and life. They also bring the death of some dreams, as humanity is no longer completely independent and may not pursue certain scientific goals, such as space exploration.
The humans remain suspicious, as the Overlords never appear in person. The Overlords' representative, Karellen, does speak with the Secretary General of the United Nations Rikki Stormgren, but is always hidden behind a pane of one way glass. To allay the inevitable suspicions of some, Karellen promises the Overlords will reveal themselves physically in fifty years, after humanity has matured and become comfortable with their presence.
Under the (mild) domination of the Overlords, Mankind enters a golden age of the greatest peace and prosperity ever known, albeit at the expense of some creativity and freedom. Stormgren, with Karellen's help, survives a kidnap attempt by subversive humans suspicious of the Overlords. Stormgren secretly harbors lingering curiosity about the real Overlord nature and smuggles a device aboard Karellen's spaceship to see behind the one-way screen that separates them. Years later he tells a questioning reporter the device failed. The novel strongly hints that the device did indeed capture an image of the Overlords, which Stormgren saw, but that Stormgren agrees with the Overlords: mankind is unready for what that image revealed.
True to their word, fifty years after arrival, the Overlords appear in person. They resemble the traditional human folklore image of demons: bipeds with large wings, horned heads, and tails. The Overlords are larger than humans, their bodies covered with a hard, black armour shell. The Overlords' resemblance to the devil of human folklore is explained as a form of racial memory. The light from Earth's sun is too harsh for them, because their planet's sun has a dimmer redder light, and, though they can breathe Earth's atmosphere, the mix of gases in their own atmosphere is more comfortable. Humankind has, however, grown accustomed to the Overlords by this time and accepts them with open arms and, with their help, creates a Utopian world.
The Overlords begin to pay a discreet interest in the human experience of the occult and psychic research. It is assumed by humans that this is part of their anthropological study of mankind. Rupert Boyce has the best collection of books on the subject in the world and when the Overlords ask to borrow them he refuses, but allows an Overlord called Rashaverak to visit him and study the books. He uses the visit to impress friends and has a party during the Overlords stay. At the party a Ouija board session is conducted, watched by Rashaverak. The answers to questions given by the board are all explicable from knowledge in the minds of the participants except when a young engineer named Jan Rodricks asks the identity of the Overlord's home planet. The boards spells out the number from a star catalog — NGS 549672. One of the participants faints and there is a commotion during which the transcription of this part of the seance disappears. It is revealed that the Overlords suspect this occurrence is the first sign of what is to come.
Jan Rodricks has however memorized the number. He becomes obsessed with finding out if the star number is correct and hatches a plan to stow away on an Overlord spaceship, which he successfully accomplishes.
Although humanity and the Overlords have developed peaceful and even friendly relations by now, the spread of equal goods and the ban on building space ships capable of travelling past the Earth's moon causes some sects to believe their innovation and independence is being suppressed and that culture is becoming stagnant. In response, those sects establish "New Athens", an island colony.
One of the colonists is George Greggson, a theatre designer. His wife Jean was the woman who fainted at the seance. They have two children, Jeffrey and Jennifer Anne. We learn that the Overlords have a special interest in the children and are watching them. This is revealed when Jeffrey's life is saved through Overlord intervention when a tsunami strikes the island.
Some ten years after the Overlords revealed themselves to humanity, human children (starting in New Athens with the Greggson children) begin displaying telepathic and telekinetic abilities and as a result, become estranged from their parents. Karellen then reveals the true purpose of why the Overlords came to Earth. They are in service to the Overmind, a cosmic mind amalgamated from ancient galactic civilizations, freed from the limitations of ordinary matter. The Overlords are not themselves capable of joining the Overmind, but the Overmind has charged them with the duty of fostering humanity's transition to a higher plane of existence and merger with the Overmind. Karellen expresses an envy of humanity; his race is trapped as they are, as they are not now capable of joining the Overmind, though he hopes they will eventually learn how to do so.
Karellen announces that the children with psychic powers will be segregated from the rest of humanity on a continent of their own, and only these children will merge with the Overmind. No more children are born; the narration subtly hints that most of the parents commit suicide, while their children evolve towards merging with the Overmind. New Athens is destroyed by its leaders, by the detonation of a nuclear bomb.
The last man alive is Jan Rodricks, a physicist, who will witness mankind's final evolutionary transformation. He stowed away on an Overlord supply ship earlier in the story in a successful attempt to travel to the Overlord home planet, which he correctly guessed orbits a star of the Carina constellation. As a physicist, Rodricks knows of the relativistic twin paradox effect: the Overlords' ships travel at a significant fraction of the speed of light, and as a result, the trip to the Overlord planet and back to Earth will only take four months in his subjective, personal time-frame, but the amount of elapsed 'objective' time will be, at minimum, 80 years, or the length of time light would take to make the similar journey, although the actual trip takes much longer. (The Overlord star system – known as NGS 549672 to astronomers on Earth – is forty light-years distant from Earth.)
When Rodricks returns from the Overlord home world, he expects no one on Earth will remember him, nevertheless, he is unprepared for the return: mankind, as he knew it, has died. About three hundred million naked young beings, physically human but otherwise with nothing in common to Man, remain on the quarantined continent. They are the final, physical form of human evolution before merging with the Overmind. Life on their continent — not only human life, but all other forms as well — has been exterminated by them. No normal children have been born and all adults have met their end through despair. The vast cities that Jan remembers are all dark and empty.
Some Overlords remain on Earth, studying the evolved children. It also is revealed here that the Overlords have met and conditioned other races for the Overmind, and that humanity is the fifth race whose apotheosis they have witnessed.
When the evolved children have grown strong enough to mentally alter the Moon's rotation and accomplish other planetary manipulations, it becomes too dangerous to remain and the Overlords prepare to leave. They offer Rodricks the opportunity of leaving with them, but he chooses to remain as witness to Earth's dissolution. Mankind's offspring have evolved to a higher existence, requiring neither a body nor a place, and thus ends mankind's childhood.
The book hints that the Overlords may have been party to Rodricks' return visit to their home planet in order to gain more information through human eyes of the final transformation of humanity. Rodricks reports, via radio, a great burning column of energy/matter ascending from Earth bearing indescribable colours and patterns. As he watches the Earth's gravity begin to decrease, the atmosphere starts escaping to space and material objects seem to dissolve around him. He reports no fear but a sense of accomplishment and completion, and then a blinding flash of light as the Earth evaporates.
The story's last scene details Karellen's final backward look at the Solar System, which becomes no more noticeable among the stars as it recedes than the loss of one small planet in the system. He is emotionally depressed, having seen yet another race evolve to the beyond, while he and his race must remain behind, limited to their current form. Despite that, he renders a final salute to mankind, considering whether or not conditioning them for the Overmind helped his goal of deciphering the evolutionary secret for his race to merge with the Overmind. He then turns away from the view, the reader presumes, to await the Overmind's next order.