We start out post plane crash, on an island full of boys ranging from ages 6 to 12, and not an adult in sight.
This is going to end well.
Well, they start with a good faith effort: Ralph and Piggy find the conch and call a meeting, kicking off the set-up of discovering the island and the other characters: What are their names? Who's going to be chief? Which one is actually evil incarnate?
Fire! No, shelters! No, pig-hunting! Oh, we set the island on fire. And missed a passing ship. There's conflict, all right: Ralph wants to make a signal fire, Piggy is all about the shelters, and Jack is looking for blood. The boy vs. boy conflict sets up the Big Ideas conflict of the book: is it human nature to create (fire, shelters, civilization) or destroy (hunting and killing)?
Even if the boys manage not to starve, burn themselves up, or die of exposure, there's still (in their minds) a good chance they'll get eaten by the beast. But the beast is more than a creature—in fact, he's not a creature at all. The beast is man's inherent darkness. Talk about complicated. As the talking pig's head so eloquently informs us, the beast isn't something that can be hunted and killed.
Any time, in any novel, when there's a Bacchic frenzy of tribal dancing, naked painted boys, and hallucinatory murder, it's the climax. This is the climax of action (murder), the psychological climax (talking, prophetic, and evil severed head), and the emotional climax (fear, disgust, and excitement all in one).
Ever since Chapter 6 when the catapult (the "bastion") shows up, we've been waiting for the other rock to drop. And it does—on Piggy. Ralph takes off through the forest, and we're biting our nails wondering if he'll make it out alive, or if the boys are all just going to end up slaughtering each other.
The denouement lasts roughly 5 seconds, when Ralph opens his eyes to see the naval officer and we realize that he's going to be okay—well, at least, alive. We're not looking forward to the therapy bills.
Ralph seeing an adult is the "phew" moment after the suspense, but just as we're breathing a sigh of relief, Golding hits us with a Message: the adult is a naval officer, and the world is at war. Okay, maybe there are no rotting pig heads on sticks—the toll of modern warfare is much worse. No happy endings here, folks.