As the story opens, the jungle on the island is severely scarred from the wreckage of the plane. Two boys, the fair-haired, charismatic Ralph, and the fat, asthmatic, thickly bespectacled Piggy, emerge from the jungle.
While they are swimming in a shallow pool inside a lagoon, Ralph discovers a beautiful conch shell. Piggy, slightly smarter, suggests he blow it as a signal for other survivors.
One by one, boys of varying ages from six to twelve appear from the jungle. Among them are several older boys, identical twins Sam and Eric (sometimes referred to as Samneric due to their lack of individual identity), the quiet but strange Roger, thoughtful Simon, and charismatic Jack Merridew, leader of the choir.
While absorbing the view, the boys come upon a wild piglet caught in some creeper vines. Jack takes out a large knife and prepares to kill the pig. He hesitates, and the pig escapes. Jack is upset by what he perceives to be Ralph’s condemnation of his hesitation. He silently vows to himself, “Next time there would be no mercy.”
Later, Ralph uses the conch to call another meeting. It is decided that whoever holds the conch shell will have the right to speak at the meetings. A small boy with a mulberry-colored birthmark obscuring half of his face receives the conch. He tells the others of a “beastie” that comes in the dark and wants to eat him. Some deny its existence, but Jack vows to hunt it when he and his hunters hunt pig for meat.
Next, the boys decide that they must make a signal fire on the mountain to attract ships to rescue them. They gather wood and use Piggy’s glasses to start the fire. In their exuberance and inexperience, they allow the fire to rage out of control and it consumes a large portion of the jungle. The small boy with the mulberry-colored birthmark disappears and is never seen again. It is implied he was killed in the fire.
Jack quickly learns the art of hunting, but still hasn’t gotten a pig. While he hunts, Ralph and Simon build poorly constructed shelters on the beach from palm trunks and fronds. Jack returns from his unsuccessful hunt, and he and Ralph clash over the decision to hunt or build the shelters.
Simon discovers a secret place in the jungle. It is a hollow completely obscured by creeper vines. He sits here, away from the others, and contemplates the beauty of the jungle.
As time passes, the boys begin to resemble less and less the civilized British schoolchildren they used to be. Their uniforms deteriorate and their hair grows long and ragged. A marked boundary begins to grow between the younger children (littluns) who play all day, and the older children (biguns) who seem to be growing divided as to their responsibilities.
Ralph, Piggy, Simon, Sam and Eric see the need for order and civilization, while Jack and his hunters become obsessed with the ideas of finding meat and protecting the littluns from the beast.
Jack introduces his hunters to the notion of camouflaging their features with red and white clay and black charcoal for hunting. This gradual masking of their identities allows them to become more ruthless and effective hunters.
Presently, the smoke from a ship passing the island is discovered, but Jack and the hunters, preoccupied with hunting, have let the signal fire they were tending go out. Jack returns from the hunt, triumphant over killing a pig and slitting its throat himself, only to be rebuffed by Ralph for neglecting the fire.
The boys clash on the matter, but eventually all share in consuming the meat. Ralph calls another meeting to deal with the situation involving the signal fire. Another littlun, Phil, speaks of his dreams of the beast. This again inspires Jack to lobby for the necessity of his hunters. He and Ralph argue again over the importance of the signal fire versus the meat. Jack declares his disgust and he and his hunters leave the meeting.
Ralph considers giving up being chief. Piggy, who fears Jack, tries to convince him not to.
That night, unseen by the castaways, there is a fight between aircraft ten miles in the sky over the island. A dead parachutist lands on the side of the mountain in a sitting position. The wind, catching in the parachute, makes the figure rock back and forth.
The boys, thinking it is the beast, argue over whether or not to approach it. The boys, led by Ralph with an angry Jack in tow, travel to the mountainside to see the beast. Jack sees the natural bridge to the island’s outcropping. He decides that the separate island, joined to the main island by a rock ledge, would make a great fort. It contains many rocks that could be rolled onto the approach path to kill enemies. He and Ralph argue again, and Jack verbally denies any further loyalty to the conch and its power.
The boys’ continued expedition to the figure on the mountain is interrupted when the boys flush a boar. Ralph wounds it when it charges him. The boar escapes, but they celebrate the encounter with another primitive blood lust dance in which Robert, pretending to be the pig, is beaten by the hunters who are overly excited by the dance. Ralph’s bravery in the face of the boar’s charge is forgotten.
As the day wanes, most of the boys have returned to the shelters, but Ralph, Jack, and Roger have pressed on and apprehensively approach the figure. The wind causes it to move and the boys see its decaying face in the darkness. They all flee.
At the next meeting, Jack and Ralph question each other’s bravery on the mountain. Jack convinces his hunters to separate themselves from the rest.
Following Piggy’s suggestion Ralph, Simon, and Samneric try to maintain a signal fire down off the mountain, away from Jack and his hunters. Jack orders his hunters to kill a pig for a feast, hoping that the roasting meat will draw the others’ loyalty away from Ralph. They kill a pig and he orders them to mount its head on a stick as a sacrifice for the beast.
Simon, who had been in his hiding place, contemplates the head of the boar that the hunters had unknowingly impaled near him. He imagines a conversation with the head, and begins to see in it the source of evil on the island. He has an epileptic seizure. He awakens, and the head again reveals itself to him as the symbol of anarchy on the island. Simon has a second seizure.
Simon awakens again and climbs the mountain to view the figure of the dead parachutist that the boys believe is the beast. He discovers that it is harmless, and that the true nature of what the boys should fear, the real beast, is symbolized by the pig’s head. He returns to tell the others.
Meanwhile, Jack and his hunters roast the pig, and the others, including Ralph and Piggy, join the feast. Ralph and Jack argue again and most of the boys side with Jack this time. Ralph tries to convince them that they need shelters, but Jack distracts them by commanding another blood lust dance. The boys become so swept up in the dance that Simon, emerging from the forest, is mistaken for the beast. All the boys, marginally including Ralph and Piggy, beat him to death. The tide sweeps his body out to sea.
Back at the shelters, Ralph, Piggy, and Samneric contemplate their roles in Simon’s death. That night, Jack and his hunters attack them and steal Piggy’s glasses for a fire.
The next day, Ralph, Samneric, and Piggy approach Castle Rock, where Jack’s tribe has gathered, to demand the return of Piggy’s glasses. Ralph wants to reestablish the power of the conch.
He and Samneric approach the hunters while Piggy and the conch stay on the stone bridge. Jack and Ralph argue again while the hunters take Samneric prisoner. Roger releases a rock they had rigged to guard the bridge. It falls on Piggy, smashes the conch, and plunges Piggy over the edge to his death.
Ralph escapes and the hunters hunt him. He hides near Castle Rock but only manages to learn that Roger has tortured Samneric into joining the hunt. Samneric now fear Roger, the sadist, more than Jack.
Eventually, the hunters corner Ralph in Simon’s old hiding place. They flush him from concealment with a fire. Ralph manages to escape to the beach with the hunters right behind.
He comes face to face with a shocked naval officer. A battle cruiser has docked in the lagoon, drawn by the smoke from Jack’s fire. The officer is appalled at the savage condition of the children. Ralph assumes responsibility for what appears to be poor leadership. Ralph begins to weep for the three dead children and the castaways’ loss of innocence.
Jack emerges onto the beach without his hunting camouflage or weapons. Only Piggy’s broken glasses on his belt give any indication of his previous savagery.
One of the littluns cannot remember his own name. The officer, embarrassed by what he mistakenly perceives to be Ralph’s undignified relief at rescue, turns away and stares at his warship in the lagoon.