I hadn't read this book in more than 50 years, but having fond memories of it, I thought I'd read it again to commemorate Clarke's passing. Turns out that it's not heavy on character development; it's one of those novels where the idea is just about everything.
The idea is that aliens show up in the sky. Their science is far in advance of ours, and they more or less take over the Earth, without ever coming down to it or allowing themselves to be seen. Clarke devotes a lot of the first third of the book to describing the changes that take place under the Overlords, as they're called. In the second third, which takes place fifty years after the first, the Overlords reveal themselves, but not their real purpose. And in the final third, we discover what that the Overlords, though they're guiding Earth, can never have for themselves the transcendent powers that are revealed in the novel's end, which is also the end of humanity's childhood.
BIG SPOILER: It's also the end of humanity, not just humanity's childhood. END OF BIG SPOILER
In spite of the fact that most of the books characters are pretty much cardboard, the last few chapters still retained the power to move me, and I can see why I have such fond memories of the novel. It's exactly the kind of thing that would have inspired me to write a lot of very bad teenage poetry. The ending, and indeed the whole book, was the kind of thing that opened up my mind to things I'd never thought about before, the kind of things that made me love SF in the first place.
Clarke tosses out a couple of ideas in one sentence that I probably overlooked fifty years ago, a couple of reasons for humanity's golden age: "The first was a completely reliable oral contraceptive; the seconed was an equally infallible method -- as certain as fingerprinting, and based on a very detailed analysis of the blood -- of identifying the father of any child." Those things were probably being discussed at the time (1953) of the books publication, but they'd have been completely new to me. I'm glad I picked this one up again.