This is Clarke's take on the "invasion from space" story, and if it's not the first iteration of the "gigantic spaceships hovering over major cities" thing it's at least the story that made it a thing.
Basic story: Overlords come to Earth and say it's time for humanity to stop being stupid. Karellen is the Supervisor of Earth, and he's here to usher in a Golden Age.
Here's the whole synopsis because I have no need to paraphrase it.
The SyFy miniseries gives us Charles Dance in what may be his first semi-sympathetic role, at least in a very long time. They slathered him with latex and makeup and other special effects to turn him into Karellen, and he did it so f-ing well it's a sight to behold; and Karellen's job is to manage humanity into extinction.
This is the problem I have with the story: the aliens win. Karellen's people work for the Overmind, a psychic entity comprised of trillions of individual minds. Given the circumstances of the story it's impossible for humans to win against such an enemy, of course, and there's no other way for the story to end.
The Overlords (as they're dubbed by the media) give us a cure for all diseases, they solve all our problems for us. There's no longer any disease, no want, no illness, no stress; there's no need for anyone to work if they don't want to. As a result, there's a huge baby boom; children are born in record numbers all over the world and there is absolutely no privation or want or anything, so besides being the largest generation they're also the healthiest. At the same time, all scientific endeavor ceases. There's no point to learning anything because the Overlords have fixed everything.
At the end of the story, all the children in the world have left Earth behind, to join the Overmind. Karellen tells the adults of Earth that no more children will be born and that humanity will now die out. In the book, the Earth fades from existence; in the miniseries, it blows up.
Here's the thing: the Overmind is essentially no different from Star Trek's Borg race. They find civilizations, they plunder them of resources, absorb what they can, and destroy the rest.
The Overmind's actions are perfectly logical. This is what a successful species does; it expands its numbers and it defeats (one way or another) its competition. It's not enough to take the children of Earth; the adults must be prevented from further breeding because that breaks humanity's spirit and prevents a highly pissed-off human race from finding the Overmind and blowing it out of existence.
It's not (as Karellen says) the final evolution of mankind, but its destruction.
The final irony of the series is that the reactionary newspaperman (played so very well by Colm Meany, who was Chief Miles O'Brien in various iterations Star Trek) was right about what the Overlords were doing on Earth.
One clue of this came near the beginning, when Karellen explains why he's there: humans are on the verge of discovering interstellar travel. What is left unsaid is that if humans can travel between the stars, the Overmind is in danger.
Karellen and his people serve the Overmind because they're the one race which cannot "evolve" to join it; but given the Overmind's complete lack of concern for other species (again, not illogical) I suspect that Karellen's people were made by the Overmind.
I last read the book decades ago, and didn't like it; having seen the miniseries--which is a very good adaptation of the story, and I highly recommend seeing it!--I now understand why.
Anyway, what I'd like to see is an adaptation of Songs of Distant Earth done to the standard set by this miniseries. Holy crap would that be a good show to watch.