Spaceships appear over Earth's major cities one day. Rather than do something stupid like, say, blow up all the cities because they're intergalactic bullies, the alien visitors (a.k.a. the Overlords) just tell the people of Earth they are in charge now. Using their advanced technology, the new alien masters bring peace to the world—whether the world wants it or not.
Years later, the U.N. Secretary-General Rikki Stormgren is working with the Overlord Ambassador Karellen to create a new World State. Some people, like the Freedom League, don't think this is such a good idea and wonder why the Overlords don't show themselves if they are so cool. But Stormgren figures those people will forget about all their beef when they see how awesome the World State will be (you know, jetpacks for all and all that jazz).
Rather than be thankful, an anti-Overlord group kidnaps Stormgren to learn as much as they can about their new alien masters. Stormgren doesn't give away any useful information, and it's not long before Karellen sends down rescue robots to save him.
Stormgren is grateful for the assist but decides he wants to solve the mystery of what the Overlords look like anyway. Teaming up with scientist Pierre Duval, Stormgren gets a device that will allow him to spy on Karellen during their meetings.
Karellen promises the world the Overlords will show themselves to the people of Earth in fifty years. Knowing he won't live that long, Stormgren uses the device to spy on the Overlord. He goes to his death content but still wondering what the Overlords have planned for mankind.
Fifty years pass, and the Overlords present themselves to the world. They look like demons—seriously, like with horns, leathery wings, red skin, the works. It's not too big a deal, though, because they usher in a Golden Age of plenty and prosperity.
Years later, at a party thrown by Rupert Boyce, George Greggson, Jean Morrel, and Jan Rodricks meet an Overlord named Rashaverak. He's at Rupert's place to study the man's library on the paranormal and supernatural.
After the party, Rupert gathers the stragglers for some slumber party shenanigans on his Ouija board. On a whim, Jan asks the board where the Overlord home world is and receives an answer. A trip to the Royal Astronomical Society confirms the answer was correct.
Jan teams up with oceanographer Professor Sullivan. They devise a plan to have Jan stow away inside a whale tableau being shipped off planet on an Overlord supply ship. The plan goes off without a hitch, and Jan races to explore the alien home world at something roughly equivalent to the speed of light.
Karellen holds a press conference, informing the world of Jan's plan. He promises that Jan will be the last human into space as mankind is not ready for what awaits them beyond the solar system.
George Greggson, Jean, and their family move to the island colony of Athens, an artistic commune designed to counter the cultural stagnation brought about by the Overlords' rule.
Jeff, George's son, is playing near the ocean one day when a tsunami surges against the islands. The boy survives, saying a voice led him to safety. George knows it was the Overlords' doing, and he's grateful but doesn't understand why.
Weeks later, Jeff begins to have mysterious dreams, and his baby sister, Jennifer, begins showing extraordinary powers like lifting her toys in midair. Rashaverak visits the family to explain to George and Jean that their children are something that is not human.
The condition spreads across the world, and Karellen holds one last press conference to explain to the world that their children are evolving to join something called the Overmind—after this generation, humanity will go extinct.
Years later, Jan returns to Earth to find he's the last man in existence. Though a major bummer, he takes the hit pretty well and hangs out on the planet perfecting his piano skills until the Overlords decide they need to leave. The children are about to transcend to the Overmind, and that means things are about to go crazy bad for the world.
Rather than go, Jan stays behind. Using a communication device, he describes to the Overlords the sight of watching the newly formed aspect of the Overmind devour the planet for sustenance.
With Earth destroyed and mankind evolved into something he can never understand, Karellen leaves the solar system behind. His job is over. And so is ours.