Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Exposition
We Come In Peace
Exposition (a.k.a. the stuff you need to know to enjoy the story) is the name of this stage's game. Basically, the initial situation occurs during the novel's first section, "Earth and the Overlords," with the first few chapters of the second section thrown in for good measure.
In this section, we are told how the Overlords come to Earth and say they'll be running the joint. We also learn how various human factions, such as the Freedom League and world nations, deal with the situation. We learn why the Overlords hide themselves from mankind as they have a face only a demonic mother could love.
Finally, the alien masters create a world of plenty and paradise but with just enough restrictions to set the stage for conflicts with certain characters' desires. These conflicts conveniently lead us to stage two.
Rising Action
Take Me Down to the Paradise City
This part is all about conflict, conflict, and more conflict—with some complication in the mix for an extra spicy kick.
The conflict comes with the golden age because some people find the Overlords' rules oppressive even if they ushered in an era of unrequited awesome. Jan's journey into space represents one of the conflicts between the Overlords' control and human ambition, while the formation of the Athens colony represents another.
Each one rebels against the Overlords in their own way and both win their battles. The war, however, is another matter.
Climax
All Your Children Belong to Us
The climax is the point of no return—everything builds to this point, and everything changes after it. Childhood's End's climax comes when Karellen reveals to the world the Overlords' true intentions: Humanity is now on the path to joining the Overmind through the transformation of its children. There is no turning back, no retreating from the path.
Falling Action
The Omega Man
The falling action is the fallout from the climax. Plot lines begin to tie themselves up, we see the fate of some characters and the story starts moving toward its conclusion.
Part of the falling action in this book is watching humanity deal with the loss of all future generations. To this effect, we see the fates of George and Jean and the results of the Overlords' manipulation in human affairs.
Jan's return to Earth is another part of this stage. As the last man on Earth, he witnesses the beginning of the end of the human race. And if the end of the human race doesn't signal the approach of a story's conclusion, then we don't know what does.
Resolution
Or Why We Can't Have Nice Things
The resolution comes with the end of humanity and the birth of another aspect of the Overmind. Jan observes humanity evolving into something more than human and the world is destroyed, ending the story for all the human characters that have not otherwise had their stories resolved.
The only characters left are the Overlords. The story leaves their fate ambiguous, but since this story was about their relationship to humanity, it's safe to say that chapter of their existences has officially come to a close.