As two cosmonauts are preparing themselves for ventures into the solar system, massive alien ships arrive overheard. “The human race was no longer alone.”
Five years later, the aliens, whom humanity has christened “the Overlords,” govern the Earth. The Overlords are minimally involved, allowing states to govern themselves so long as they are not cruel. To the few countries who ever defy them, they are not violent, but merely demonstrate their power—in one case by blotting out the sun for thirty minutes, which frightens their subjects into submission. The Overlords have never emerged from their ships and have never been seen by human eyes. Their leader, Karellen, who refers to himself as Earth’s “supervisor,” speaks with the Secretary-General of the United Nations, Rikki Stormgren, aboard his ship once a week to discuss matters of government. Stormgren is the single point of contact between humanity and the Overlords.
Stormgren meets with the leader of the Freedom League, a group opposed to the Overlords’ presence based on religious and libertarian grounds, even though most of humanity sees the Overlords as benevolent and seem content to live under their governance. The Freedom League also resent the Overlords’ secrecy, since the Overlords will not reveal their plans or even their physical appearance to humanity.
Stormgren relays the Freedom League’s concerns to Karellen during their next meeting, who says he will take the request for transparency to his superiors, although he knows what the answer will be. Some days later, Stormgren is kidnapped by an extremist element of the Freedom League and held hostage underground. His kidnappers are trying to discover the true nature of Karellen and devise a plan for Stormgren to attend his weekly meeting with a hidden observational device. Stormgren refuses and is promptly rescued by Karellen, whom he realizes used him as bait to draw out and identify the resistance leaders. Stormgren is vaguely annoyed at being used and begins considering carrying out the plan to observe Karellen himself.
Stormgren becomes obsessed with the idea and enlists the help of a physicist to help him finally see the Overlord. Together, and with a little covert scanning, they realize that what Stormgren had assumed was a viewing screen in the Overlords’ conference room is actually a sheet of one-way glass that Karellen is sitting behind in an adjacent room. In their next meeting, Stormgren has brought a flashlight to shine through the glass and hopefully see Karellen’s physical form. Karellen tells him that he has been given an answer to the Freedom League’s request for the Overlords to show themselves: the Overlords will emerge from their ships in fifty years. Stormgren is fond of Karellen, and his will to go through with the scheme to observe Karellen briefly falters. But when he realizes that he will be dead before the Overlords emerge, he shines the light through the glass. It seems that Karellen was wise to his plan, but also sympathetic to it. Stormgren only catches a glimpse of Karellen exiting through a doorway, but it is enough. However, he chooses to keep what he has seen a secret until his death.
Fifty years pass and the Overlords fulfill their promise to emerge from their ships. Karellen steps out first before a crowd of onlookers, holding two human children who entered his ship to meet him. The Overlord looks like the quintessential picture of the Devil: 12 feet tall; black, armored skin; leathery wings; barbed tail; short horns protruding from his head. However, in those fifty years, the Earth has become a utopian world-state. Everyone has access to all the necessities of life for almost no cost, sexual ethics have loosened, and education has become a lifelong and leisurely pursuit.
Years later, partners George Greggson and Jean Morrel are attending a party hosted by a wealthy friend and enthusiast, Rupert Boyce, who has amassed the world’s largest library on paranormal happenings. While exploring Rupert’s house, George and Jean stumble upon the Overlord, Rashaverak, who is busily reading through every book in Rupert’s library. During the party, George meets Jan Rodricks, a young enterprising scientist who is frustrated that the Overlords will not reveal where they have come from and have prohibited humanity from space travel. As the party is winding down, Rupert organizes several guests around a Ouija board as a party game, claiming that he doesn’t understand it but it seems to work. After several superficial questions are answered by the board, Jan asks what star the Overlords came from. The board lists off a string of numbers and letters, which the other guests assume is meaningless but Jan and Rashaverak both recognize as coordinates in space. As soon as the board answers, Jean suddenly faints, frightening George who promptly takes her home and asks her to marry him.
Rashaverak reports to Karellen on the contents of Rupert’s library and what happened with the Ouija board, advising that a close watch should be kept on Jean, since she seems to be channeling something.
Jan, determined to know more about the Overlords, comes up with a plan to stow away aboard one of their ships by hiding in a taxidermied whale specimen, which a friend of Rupert’s, Professor Sullivan, is currently readying to send to the Overlords’ home planet for them to study. Jan convinces Sullivan to help him, and his plan succeeds. The whale is delivered to the Overlords’ ship with Jan inside, and it leaves for its homeworld. Since the ship is traveling at near-light speed, Jan will only age two months during the trip, but forty years will pass on Earth.
Although, in the Overlords’ utopian Earth, everyone is well-fed, leisurely, and safe, the arts are in decline. Much art is being made, but without the stimulus of true suffering, hardship, and sorrow, there is nothing to give any of it substance. Humanity is left looking over its shoulder at the artistic greatness of past eras, the likes of which will never exist again.
In an attempt to solve this, a colony named New Athens is built upon a volcanic island. The colony is an artists’ commune, attempting to re-stimulate humanity’s dulled imagination by fostering competition between individuals and shirking some modern amenities to create a simpler, less distracted lifestyle. George, who works in theater and is now married to Jean, decides that they shall move to New Athens along with their son, Jeffrey, and newborn daughter, Jennifer.
One day, while Jeffrey is alone on the shore, a tsunami hits the island. Though it seems he should have died, an Overlord’s voice (though he does not know it at the time) appears in his head and guides him to safety. Shortly after, an Overlord Inspector comes to New Athens under the pretense of wanting to investigate what the artists are working on, though his real intention is to observe Jeffrey. After the Overlord leaves, Jeffrey begins having vivid dreams every night of other planets, which remain in his head like actual memories. Meanwhile, Jennifer, still an infant, begins exhibiting psychic powers as well—she seems almost catatonic, eyes always closed, and yet is able to levitate objects around her. Rashaverak meets with George and Jean, explaining to them that their children are no longer technically human and will not remain with them much longer. All of the children on Earth start showing similar symptoms, standing with their eyes closed for hours at a time.
Karellen makes his final broadcast speech to Earth, where he finally explains the Overlords’ purpose in full. They have been sent by a being called the Overmind—a universal collective consciousness that absorbs sentient races when they have achieved a certain level of development—to supervise humanity’s next developmental phase. The children of Earth will soon merge into the Overmind and transcend their biological forms. The rest of the Earth is tragically doomed to die, comforted only by the fact that humanity is living on in a new form. As the Overlords prepare the children for their exit, the people of Earth die out by suicide, small warfare, or, in the case of New Athens, volcanic explosion.
On the Overlord ship bound for their homeworld, Jan reveals himself to the Overlord crew. When he arrives on the Overlord planet, scientists interview him and run tests on him, and then let him explore their world for several weeks with the help of a tutor. Most of what Jan sees he cannot understand, including a fiery mountain that he later guesses may be some sort of manifestation of the Overmind. He is put back on a ship returning to Earth, and he arrives eighty Earth-years after his initial departure. Humanity is dead. Millions of children have been gathered on one continent, awaiting the Overmind. The Overlords prepare to leave, for it is too dangerous for them to stay, but Jan decides to remain and report to Karellen what he sees. As the last human being to ever live, Jan watches as the Overmind absorbs the children into itself. As it does, the children destroy the planet and consume its emitted energy.
Karellen observes this from his ship, listening to Jan’s final narration of it. He thinks of all the humans he knew, looks at Earth’s sun one last time, and returns home, having gained a little more knowledge of his master.
