In the midst of a raging war, a plane evacuating a group of schoolboys from Britain is shot down over a deserted tropical island. Two of the boys, Ralph and Piggy, discover a conch shell on the beach, and Piggy realizes it could be used as a horn to summon the other boys. Once assembled, the boys set about electing a leader and devising a way to be rescued. They choose Ralph as their leader, and Ralph appoints another boy, Jack, to be in charge of the boys who will hunt food for the entire group.
Ralph, Jack, and another boy, Simon, set off on an expedition to explore the island. When they return, Ralph declares that they must light a signal fire to attract the attention of passing ships. The boys succeed in igniting some dead wood by focusing sunlight through the lenses of Piggy's eyeglasses. However, the boys pay more attention to playing than to monitoring the fire, and the flames quickly engulf the forest. A large swath of dead wood burns out of control, and one of the youngest boys in the group disappears, presumably having burned to death.At first, the boys enjoy their life without grown-ups and spend much of their time splashing in the water and playing games. Ralph, however, complains that they should be maintaining the signal fire and building huts for shelter. The hunters fail in their attempt to catch a wild pig, but their leader, Jack, becomes increasingly preoccupied with the act of hunting.
When a ship passes by on the horizon one day, Ralph and Piggy notice, to their horror, that the signal fire—which had been the hunters' responsibility to maintain—has burned out. Furious, Ralph accosts Jack, but the hunter has just returned with his first kill, and all the hunters seem gripped with a strange frenzy, reenacting the chase in a kind of wild dance. Piggy criticizes Jack, who hits Piggy across the face. Ralph blows the conch shell and reprimands the boys in a speech intended to restore order. At the meeting, it quickly becomes clear that some of the boys have started to become afraid. The littlest boys, known as "littluns," have been troubled by nightmares from the beginning, and more and more boys now believe that there is some sort of beast or monster lurking on the island. The older boys try to convince the others at the meeting to think rationally, asking where such a monster could possibly hide during the daytime. One of the littluns suggests that it hides in the sea—a proposition that terrifies the entire group
Not long after the meeting, some military planes engage in a battle high above the island. The boys, asleep below, do not notice the flashing lights and explosions in the clouds. A parachutist drifts to earth on the signal-fire mountain, dead. Sam and Eric, the twins responsible for watching the fire at night, are asleep and do not see the parachutist land. When the twins wake up, they see the enormous silhouette of his parachute and hear the strange flapping noises it makes. Thinking the island beast is at hand, they rush back to the camp in terror and report that the beast has attacked them.
The boys organize a hunting expedition to search for the monster. Jack and Ralph, who are increasingly at odds, travel up the mountain. They see the silhouette of the parachute from a distance and think that it looks like a huge, deformed ape. The group holds a meeting at which Jack and Ralph tell the others of the sighting. Jack says that Ralph is a coward and that he should be removed from office, but the other boys refuse to vote Ralph out of power. Jack angrily runs away down the beach, calling all the hunters to join him. Ralph rallies the remaining boys to build a new signal fire, this time on the beach rather than on the mountain. They obey, but before they have finished the task, most of them have slipped away to join Jack.Jack declares himself the leader of the new tribe of hunters and organizes a hunt and a violent, ritual slaughter of a sow to solemnize the occasion. The hunters then decapitate the sow and place its head on a sharpened stake in the jungle as an offering to the beast. Later, encountering the bloody, fly-covered head, Simon has a terrible vision, during which it seems to him that the head is speaking. The voice, which he imagines as belonging to the Lord of the Flies, says that Simon will never escape him, for he exists within all men. Simon faints. When he wakes up, he goes to the mountain, where he sees the dead parachutist. Understanding then that the beast does not exist externally but rather within each individual boy, Simon travels to the beach to tell the others what he has seen. But the others are in the midst of a chaotic revelry—even Ralph and Piggy have joined Jack's feast—and when they see Simon's shadowy figure emerge from the jungle, they fall upon him and kill him with their bare hands and teeth.
The following morning, Ralph and Piggy discuss what they have done. Jack's hunters attack them and their few followers and steal Piggy's glasses in the process. Ralph's group travels to Jack's stronghold in an attempt to make Jack see reason, but Jack orders Sam and Eric tied up and fights with Ralph. In the ensuing battle, one boy, Roger, rolls a boulder down the mountain, killing Piggy and shattering the conch shell. Ralph barely manages to escape a torrent of spears.
Ralph hides for the rest of the night and the following day, while the others hunt him like an animal. Jack has the other boys ignite the forest in order to smoke Ralph out of his hiding place. Ralph stays in the forest, where he discovers and destroys the sow's head, but eventually, he is forced out onto the beach, where he knows the other boys will soon arrive to kill him. Ralph collapses in exhaustion, but when he looks up, he sees a British naval officer standing over him. The officer's ship noticed the fire raging in the jungle. The other boys reach the beach and stop in their tracks at the sight of the officer. Amazed at the spectacle of this group of bloodthirsty, savage children, the officer asks Ralph to explain. Ralph is overwhelmed by the knowledge that he is safe but, thinking about what has happened on the island, he begins to weep. The other boys begin to sob as well. The officer turns his back so that the boys may regain their composure.