The novel is set on an unidentified deserted island. The island is large enough for the boys to not know it is an island at first, and for there to be multiple distinct settings for events to occur (the mountaintop, Castle Rock, the jungle). However, the boys are the only humans on the island until the very end of the novel. The setting drives the plot—the island is deserted, and therefore the boys must fend for themselves.
In Chapter 1, Ralph says this to the other boys:
We’re on an island. We’ve been on the mountain top and seen water all round. We saw no houses, no smoke, no footprints, no boats, no people. We’re on an uninhabited island with no other people on it.
At first, the boys believe this will make things more fun. But Ralph eventually wishes adults were around to take charge, tell them what to do, and reassure them.
Adventure novels about shipwrecks on deserted islands often present the island as a natural paradise where the characters can use their intelligence and the island's resources to survive and even thrive. However, the deserted island setting can also be much darker: the characters might struggle for survival. In Lord of the Flies, both qualities seem to be true of the island. There is enough food, wood, fresh water, and shelter for the boys to survive and play, and they even take advantage of the natural environment to design rudimentary tools such as spears. But the island is also inhospitably hot and terrifyingly dark at night. Readers might see the island as Edenic or prelapsarian; in other words, the island is an undisturbed natural environment in which the boys return to a primitive state, like the Bible's Garden of Eden. Because the setting allows the boys to exist "without civilization," they act as they would "in nature." This setting allows Golding to articulate claims about human nature.
Golding's descriptions of the island are rich and beautiful, even when they are meant to evoke dread or fear. He uses metaphors, similes, personification, and imagery to make the island defined and memorable for the reader. The beauty and harshness of the island intertwine in the novel.