In the midst of a wartime evacuation, a British aeroplane crashes on an isolated island in a remote region of the Pacific Ocean. The only survivors are boys in their middle childhood or preadolescence. A fair-haired boy named Ralph and a fat boy nicknamed Piggy find a conch shell, which Ralph uses as a horn to gather the survivors. Ralph immediately commands authority over the other boys using the conch, and is elected their "chief". He establishes three goals for the boys: to have fun, to survive, and to constantly maintain a smoke signal that could alert passing ships. Ralph, a red-haired boy named Jack, and a quiet boy named Simon use Piggy's glasses to create a signal fire.
The semblance of order deteriorates as the boys grow lazy and ignore Ralph's efforts to improve life on the island. They become paranoid about an imaginary monster called "the beast". Ralph fails to convince the boys that no beast exists, while Jack gains popularity by declaring that he will personally hunt and kill the monster. At one point, Jack takes the boys to hunt a wild pig, including the boys who were meant to watch the signal fire. The smoke signal goes out, failing to attract a ship that was passing by the island. Ralph angrily confronts Jack and considers relinquishing his role as leader, but is persuaded not to do so by Piggy.
One night, an air battle occurs near the island and the body of a fighter pilot drifts down in a parachute. Twin boys Sam and Eric mistake the corpse for the beast. When Ralph and Jack investigate with another boy, Roger, they flee in terror, believing the beast is real. Jack tries to turn the others against Ralph, and goes off alone to form his own tribe, with most of the other boys gradually joining him.
Jack and his followers set up an offering to the beast in the forest: a pig's head, mounted on a sharpened stick and swarming with flies. Simon, who often ventures into the forest alone, has an imaginary dialogue with the head, which he dubs the "Lord of the Flies". The head tells Simon that there is no beast on the island, and predicts that the other boys will turn on Simon. That night, Ralph and Piggy visit Jack's tribe, who have begun painting their faces and engaging in primitive ritual dances. When Simon realises that the beast is only a dead pilot, he rushes to tell Jack's tribe, but the frenzied boys (including Ralph and Piggy) mistake Simon for the beast and beat him to death.
Jack and his tribe steal Piggy's glasses, the only means of starting a fire. Ralph goes to Jack's camp with Piggy, Sam, and Eric to confront him and retrieve the glasses. Roger triggers a trap that kills Piggy and shatters the conch. Ralph manages to escape, but Sam and Eric are forced to join Jack's tribe.
That night, Ralph secretly confronts Sam and Eric, who warn him that Jack plans to hunt him. The following morning, Jack's tribe sets fire to the forest. Ralph narrowly escapes the boys and the fire, and finally falls in front of a uniformed adult – a British naval officer who has landed on the island to investigate the fire. Ralph, Jack, and the other boys erupt into sobs over the "end of innocence". The officer expresses his disappointment at seeing the boys exhibiting such feral, warlike behaviour, then turns, "moved and a little embarrassed," to stare at his cruiser waiting offshore.
