In the late 20th century, the United States and the Soviet Union are competing to launch the first spacecraft into orbit when alien spaceships suddenly position themselves above Earth's principal cities. After one week, the aliens announce they are assuming supervision of international affairs, to prevent humanity's extinction. They become known as the Overlords. In general, they let humans go on conducting their affairs in their own way, although some humans are suspicious of the Overlords' benign intent, as they never allow themselves to be seen.
The Overlord Karellen, the "Supervisor for Earth," periodically meets with Rikki Stormgren, the Secretary-General of the United Nations. Karellen tells Stormgren that the Overlords will reveal themselves in 50 years, when humanity will have become used to their presence. When the Overlords finally reveal their appearance, they resemble the traditional Christian folk images of demons, with cloven hooves, leathery wings, horns, and barbed tails. Humankind enters a golden age of prosperity at the expense of creativity.
The Overlords are interested in psychic research, which humans suppose is part of their anthropological study. Rupert Boyce, a prolific book collector on the subject, allows one Overlord, Rashaverak, to study these books at his home. To impress his friends with Rashaverak's presence, Boyce holds a party, during which he makes use of a Ouija board. Jan Rodricks, an astrophysicist and Rupert's brother-in-law, asks the identity of the Overlords' home star. The Ouija board reveals a number which Jan recognizes as a star-catalog number and learns that it is consistent with the direction in which Overlord supply ships appear and disappear. Jan stows away on an Overlord supply ship and travels 40 light years to their home planet.
Well over a century after the Overlords' arrival, human children, beginning with the Greggsons', begin to display clairvoyance and telekinetic powers. Karellen reveals the Overlords' purpose: they serve the Overmind, a vast cosmic intelligence, as a kind of "bridge species", fostering other races' eventual union with it.
As Rashaverak explains, the time of humanity as a race composed of single individuals with a concrete identity is coming to an end. The children's minds reach into each other and merge into a single vast group consciousness. For the transformed children's safety, they are segregated on a continent of their own. No more human children are born and many parents die or commit suicide. When Jan Rodricks returns to Earth, he finds an unexpectedly altered planet. Humanity has effectively become extinct and he is now the last man alive.
Some Overlords remain on Earth to study the children from a safe distance. When the evolved children mentally alter the Moon's rotation and make other planetary manipulations, it becomes too dangerous to remain. The departing Overlords offer to take Rodricks with them, but he chooses to stay to witness Earth's end and transmit a report of what he sees.
