It is the late twentieth century. Both the United States and the Soviet Union are about to enter outer space using nuclear-powered spaceships. Just as both countries close in on the achievement of space travel, a number of giant alien spaceships come down over every major city in the world.
Five years later, these aliens, known as the Overlords, have taken control of the entire world. Their flagship is based over New York and is managed by Karellen, the Overlord who is charge of Earth-related affairs. The humans know Karellen as "the Supervisor." Karellen uses the Secretary-General of the United Nations, Stormgren, as his liaison to the human race. Once every few weeks, Karellen meets with Stormgren inside his ship. However, neither Stormgren nor any other human has ever seen what an Overlord looks like.
Stormgren is attempting to placate the Freedom League, a group that opposes the Overlords' dominion, no matter how friendly the aliens seem to be. A radical fringe group of the Freedom League kidnaps Stormgren; Karellen uses Stormgren as bait so he can keep track of these radicals. The radicals do give Stormgren the idea to use a scanning device to try and find out what is inside the room where he and Karellen meet. Stormgren then meets with Karellen, who announces that the Overlords will reveal themselves in fifty years. Stormgren then uses the scanner and discovers that Karellen is on the other side of the blank "viewscreen"--it's just a piece of one-way glass. Later, as he is handing over the paperwork for the World Federation, a new world government, Stormgren uses a flashlight to sneak a glimpse of Karellen.
Fifty years later, the Earth has greatly changed. It has become a utopia, where everyone has a place to live and enough food to eat. The Overlords have altered the world and raised the standard of living for everyone. At this time, the Overlords reveal themselves: they look almost exactly like the Devil, with dark black skin, giant wings, horns on their heads, and a barbed tail.
A few years later, George Greggson and Jean Morrel attend a party held by Rupert Boyce, an amateur scholar of the paranormal. While there, George and Jean run into Rashaverak, an Overlord making use of Boyce's research library. George also meets Jan Rodricks, a restless astronomer who is annoyed that the Overlords will not let mankind explore space. During the party, Boyce organizes a séance using a mechanized Ouija board. The board gives surprisingly accurate answers, and when Jan asks where the Overlords' homeworld is, it gives an exact star number. Jean faints after this incident. This fainting fit worries George, and he proposes marriage to Jean. Meanwhile, Jan takes the information he has learned and confirms it: the Overlord supply ships take off in the direction of the star given by the Ouija board. Rashaverak reports to Karellen and informs him that Jean should be watched, as she may be "the most important human alive."
After speaking again with Boyce, Jan gets an idea of how to sneak aboard an Overlord vessel. He meets with Sullivan and makes a plan to stow away inside a giant sperm whale that Sullivan is stuffing for the Overlords. The plan succeeds, and soon Jan is on his way to the Overlord homeworld. While the trip will seem like only a few months to him, due to time dilation at the speed of light eighty years will pass on Earth.
Back on Earth, George and Jean begin their life together. Eventually, George comes to believe he is stuck in a rut, and the couple decide to go to New Athens, an experimental community based on an island. The members of the community are primarily artists, and the hope is that they will reinvigorate the stagnating imagination of humanity. George and Jean take their two kids, Jeffrey and Jennifer, with them. The family adjusts to life on New Athens fairly well, but one day, a tsunami hits the island while Jeffrey is down at the beach. Jeffrey mysteriously survives and says that he heard a voice in his head that told him where to run.
A few weeks later, an Overlord visits New Athens. He tells the community he is just performing a routine inspection, but actually he is checking on Jeffrey. A few weeks after his visit, Jeffrey begins to have strange dreams, where he is walking across alien landscapes. His parents don't know what to make of the dreams, but the Overlords recognize the planets he describes as real worlds. George meets with Rashaverak, who tells him that his children aren't human-- they're something beyond humanity.
Jennifer, even though she is a baby, soon begins to exhibit powers herself--and her powers are even greater than those of Jeffrey. Soon all the children of the world are showing signs of strange mental powers. Karellen addresses all of mankind one last time, telling them that their children are taking a new evolutionary step. This is why the Overlords were sent to Earth--sent by the Overmind, a powerful being of thought and energy that absorbs and assimilates races once they have matured enough. Ironically, this is a step that the Overlords have never been able to make and may never be able to. The rest of humanity, however, is doomed. While the Overlords round up the children, many people decide to kill themselves. Jean and George die as New Athens blows itself up in a nuclear explosion.
Back on the Overlords spaceship, Jan arrives on the Overlords' homeworld. He is subjected to many tests by the curious Overlords and asked many questions. He is allowed to explore their cities a little bit, and he sees a strange mountain that may be an avatar of the Overmind. Finally, however, he is shipped back home. Nothing prepares him for what he sees: the Earth is deserted, except for three hundred million strange children. They have slowly been growing in power and ability, and their individual personalities have all but vanished. Soon, the Overmind will arrive, and the Earth will be destroyed as the children join the Overmind. While the Overlords prepare to leave, Jan decides to stay on Earth. As he watches, the Overmind arrives and absorbs the children. In the process, the children consume the Earth's crust, causing it to explode. The children consume all of the Earth for energy as they complete their transformation into the Overmind. From his ship, thousands of kilometers away, Karellen watches the end of humanity. He then awaits his next call from the Overmind and continues to explore its secrets.