It's an early image in the novel that stays with people, and has been intentionally or unintentionally copied in such TV and movie works as the original V miniseries, Independence Day, and Skyline. Absolutely enormous alien spacecraft show up one day over the major cities of the Earth. And then the aliens start to talk to us, though they refuse to show themselves to anyone.
The aliens are soon known as the Overlords. With their guidance and technological expertise, Earth soon enters a Golden Age of peace and prosperity, though there are a few growing pains. The first section of the book explores these early stages through the person of the United Nations President who becomes the only liaison with the Overlords allowed to enter their ships. But even he doesn't know what an Overlord looks like. When he finds out, he keeps the secret. But even that secret will turn out not to be what it seems.
Once the Overlords finally start mingling with humanity, 50 years after their arrival, they continue to help run the Earth. And while they're at it, they keep humanity from pursuing anything like a space program. Why? Are the stars really not meant for Man, as one character opines? And why are the Overlords so curious about tales of psychic phenomena?
Well, eventually we'll learn. Some very cold winds begin to blow as the novel approaches its end. One of the oddities of the original publication, Clarke notes in his afterword to this revised 1990 version, was that Clarke put a disclaimer at the front to note that he didn't agree with one of the book's central tenets (The stars are not meant for man). And he also notes that by 1990 he no longer really believed that evolution would feature some of the paranormal powers shown here. Clarke had been hoaxed by that great hoaxing spoon-bender Uri Geller in the interim, and subsequently learned how he had been hoaxed.
When people talk of cosmic science fiction, this novel would be one of those things they'd be talking about. It's a novel about the fate of humanity and the fate of the Earth. It's also a novel about evolution and extinction -- including the extinction of the individual consciousness. And watching over it all, those enigmatic Overlords, who have become by the end peculiarly sympathetic and perhaps even heroic in the face of their own insignificance. Highly recommended.