A group of English schoolboys are marooned on a jungle island with no adults after their plane is shot down in the middle of a war. Two of the boys, Ralph and Piggy find a conch shell. Ralph blows into it like a horn, and all the boys on the island assemble. At the assembly, a boy named Jack mocks Piggy for being fat and runs against Ralph to become chief of the group. Ralph wins the election, and declares Jack the leader of the group's hunters. Soon after, Ralph, Jack, and another boy named Simon explore the island and discover wild pigs.
At a second assembly, the boys set up rules to govern themselves. The first rule is that whoever wants to speak at an assembly must hold the conch. At the meeting, one young boy claims he saw a "beastie" in the jungle, but Ralph dismisses it as just the product of a nightmare. Ralph then suggests that they build a signal fire at the top of a mountain so any passing ships will see its smoke and rescue them. The boys use Piggy's glasses to light the fire, but they're careless, and accidentally set part of the forest on fire. The boy who saw the beastie vanishes during the fire and is never seen again.
Time passes. Tensions rise. Ralph becomes frustrated when no one helps him build shelters. Lots of boys goof off, while Jack obsesses about hunting and takes every opportunity to mock Piggy, who is smart but weak. Simon, meanwhile, often wanders off into the forest to meditate. The rivalry between Ralph and Jack erupts when Jack forces the boys who were supposed to watch the signal fire come hunting with him. They kill their first pig, but a ship passes while the signal fire is out, which causes a tremendous argument between Ralph and Jack.
Ralph calls an assembly hoping to set things right. But the meeting soon becomes chaotic as several younger boys talk about the beast. Now even the bigger boys are fearful. That night, after a distant airplane battle, a dead parachutist lands on the mountaintop next to the signal fire. The boys on duty at the fire think it's the beast. Soon Ralph and Jack lead an expedition to search the island for the beast. While searching, they find a rock outcropping that would make a great fort, but no beast. Tempers between the two boys soon flare up, and they climb the mountain in the dark to prove their courage. They spot the shadowy parachutist and think he's the beast.
The next morning, Jack challenges Ralph's authority at an assembly. Ralph wins, but Jack leaves the group, and most of the older boys join him. Jack's tribe paint their faces, hunt, and kill a pig. They then leave its head as an offering to the beast. Simon comes upon the head, and sees that it's the Lord of the Flies—the beast within all men. While Jack invites everyone to come to a feast, Simon climbs the mountain and sees the parachutist. When Simon returns to tell everyone the truth about the "beast," however, the boys at the feast have become a frenzied mob, acting out a ritual killing of a pig. The mob thinks Simon is the beast and kills him.
Jack's tribe moves to the rock fort. They steal Piggy's glasses to make fire. Ralph and his last allies, Piggy and the twins named Samneric, go to get the glasses back. Jack's tribe captures the twins, and a boy named Roger rolls a boulder from the fort that smashes the conch and kills Piggy. The next day the tribe hunts Ralph, setting fire to the forest as they do. He evades them as best he can, and becomes a kind of animal that thinks only of survival and escape. Eventually the boys corner Ralph on the beach where they first set up their society when they crash landed on the island. But the burning jungle has attracted a British Naval ship, and an officer is standing on the shore. The boys stop, stunned, and stare at the man. He jokingly asks if the boys are playing at war, and whether there were any casualties. When Ralph says yes, the officer is shocked and disappointed that English boys would act in such a manner. Ralph starts to cry, and soon the other boys start crying too. The officer, uncomfortable, looks away toward his warship.