A young hero falls under the shadow of the dark power
There isn't really a single hero or dark power in Childhood's End. No Frodo versus Sauron. No Luke Skywalker versus the Emperor.
Instead, the dark power is the Overlords while mankind itself takes the place of the hero. The Overlords come in their spaceships and very literally cast their shadow over the people of Earth. Humans lose power over their destiny and, in a way, their free will. The Overlords tell people how to live and govern themselves. Sure, some people—even our main character for Part 1, Stormgren—think it's swell. And the Overlords certainly aren't evil masterminds who stroke cats menacingly while contemplating the launch of a nuclear weapon from their volcano base.
Yet none of this changes the fact that mankind is under the power of the Overlords, utterly and fully.
All goes reasonably well
Things still seem to go pretty well for mankind all the same. The World State brings peace to all the countries of the world—and yeah, the Overlords look like demons, but their rules and technological improvements enhance people's lifestyles, ushering in a golden age of health, leisure, and mental well being. But one person's golden age is another's darkness, and it isn't long before we meet a few of these paradise killjoys.
Hero is seen imprisoned in state of living death
While not everyone sees the Overlords' rule as a "state of living death," there are some who do, most notably Jan Rodricks and Ben Salomon.
Jan sees the Overlords' ruling that no human can enter space as a type of imprisonment. So he tricks the Overlords into taking him aboard their spaceship and transporting him to their home world, so he might further scientific knowledge and curb his own wanderlust.
Ben Salomon sees the decline of the arts as a type of decay, both culturally and spiritually. He fights back against the Overlords by creating the colony of Athens, a haven for artistic experimentation and development.
Seems the dark power has completely triumphed
This stage picks up when the children of the world, most notably Jeff and Jennifer, begin their transformation. It seems as though the Overlords have won: They've manipulated humanity to take an evolutionary path that they themselves did not choose.
But wait a second… A plot twist reveals the Overlords themselves are doing the bidding of an entirely other species called the Overmind. So this is not the Overlords' triumph at all. In fact, it's suggested that the alternative—becoming like the Overlords—is a fate far more weaksauce than the alternative.
So, did the dark power triumph or not? Well, it's hard to say. It all depends on what you, the individual reader, think about the next stage.
Miraculous redemption or rebirth
We're talking very literal rebirth in Childhood's End, to be clear.
The remains of humanity transform, or are reborn, into something entirely alien to the Overlords. They go to join the Overmind, ultimately succeeding in a task the Overlords can never hope to achieve. Meanwhile, the Overlords continue working for the Overmind—what humanity thought of as a type of master has now truly been revealed as the slave.
So does the dark power really triumph? Or does the miraculous rebirth mean humanity ultimately won the day? What do you think?