Narrated in the first person by one of the main characters, graphic artist Ed Gentry, the novel begins with four middle-aged men in a large Georgia city planning a weekend canoe trip down the fictional Cahulawassee River in the north Georgia wilderness. The river valley will soon be flooded by a dam to create a reservoir. Besides Ed, the protagonists are insurance salesman Bobby Trippe, soft drink executive Drew Ballinger, and landlord Lewis Medlock, an outdoorsman who is the driving force behind the canoe trip.
The men drive into the mountains with two canoes. At a gas station in a mountain hamlet, Drew gets out his guitar and plays a duet with Lonnie, a banjo-playing, mentally deficient, inbred albino boy who is apparently a musical savant. After arranging with some local mechanics, the rough and forbidding Griner brothers, to drive the foursome's cars down to the fictitious town of Aintry, where the canoe voyage will end two days later, the men put into the river and begin their journey. After they shoot some initial rapids and the evening approaches, Ed reflects on the isolation into which the group has now voyaged.
The following morning, Ed awakes early and goes hunting with his bow and arrow. Sighting a deer, he shoots but misses because he loses his nerve at the last moment. Lewis is disappointed and Ed grows irritated at his survivalist mentality. After breaking camp, Ed and Bobby set out in one canoe slightly ahead of Lewis and Drew. After spending a night in the camp, Bobby has changed his mind about the outing and is chafing at Lewis' directions, so Ed takes him as a canoe partner to keep the two apart.
Later in the day, two mountain men, one of them toothless and carrying a shotgun, step out of the woods and accost Ed and Bobby. The men force them into the woods, tie Ed to a tree and cut him with his own knife; then, in the book's most infamous scene, one mountain man sodomizes Bobby. During this violation, the Toothless Man is still holding Ed hostage at gunpoint. As a result, the men untie Ed, and the other mountain man, handing his partner the shotgun, prepares to force Ed to perform oral sex on him. At the moment of the shotgun transfer, Lewis, hidden in the woods, shoots Bobby's assailant with an arrow. Ed wrestles the gun from the other man, who runs into the woods, and watches as the wounded hillbilly dies.
The men have a heated discussion about what to do. Lewis wants to bury the body, arguing that if they inform the police they might be tried by a jury consisting of the dead man's relatives. Drew wants to turn the body over to police in Aintry. Bobby, humiliated and traumatized, physically attacks the corpse and then agrees with Lewis, adding "I don't want this getting around." Drew desperately tries to persuade Ed to "do the right thing," but Ed ignores his pleas and sides with Lewis. The men bury the body and take to their canoes, with Ed and Drew teamed up again. That evening they enter a high gorge with powerful rapids, where Drew falls out of his canoe; both canoes are capsized, and Lewis breaks his leg. The party's wooden canoe breaks up in the rapids and will emerge again as a plot point near the conclusion.
Lewis declares that Drew was shot by the escaped mountain man. Ed is less certain, but realizes that if the mountain man is indeed at the top of the gorge, he can shoot them all if they enter the river again. Ed decides to climb up the gorge and kill the mountain man with his bow and arrow.
Ed briefs Bobby carefully about taking Lewis downriver in the remaining canoe at first light in order to avoid being shot. Ed then makes the grueling climb to the top of the gorge, climbs a tree, and waits for the rifleman. Early the following morning, a man appears, and as he spots the tree in which Ed is hidden, Ed shoots him. The rifleman fires at almost the same time, missing Ed, but knocking him from the tree. He is gored in the side by one of his own arrows as he hits the ground. He then tracks the rifleman and finds his dead body in a small clearing. He returns the body to the top of the cliff. As he does, he sees the canoe with Lewis and Bobby in it moving out into the river in the full light. Ed tries to identify the body, specifically inspecting the mouth to see if it is the toothless man who fled the scene of Bobby's rape. To his horror, he discovers a set of teeth but realizes it is a partial set of dentures. He is unsure whether he recognizes the dead man or not.
Ed then lowers the body down the cliff and descends the rope himself, but it is not long enough to reach to the bottom. As Ed climbs down it breaks; he kicks against the cliff and propels himself into the river. The dead body lands face first on a large rock, destroying the dead man's facial features. Ed asks Bobby to try and identify him as one of the rapists, but his face is too mangled and Bobby cannot. The men never know if Ed has killed one of the rapists, a different hostile mountain man who perhaps shot Drew, or just an unrelated hunter out for a stroll along the top of the gorge. After Ed castigates Bobby for not following his plans, the pair weight and sink the corpse. Ed throws into the river all the evidence of the encounter: his bow, arrows, and the mountain man's gun. Ed, Bobby, and the badly injured Lewis then continue the journey in the remaining canoe.
Below the gorge, they find Drew's body. Lewis examines his head and confirms it was grazed by a rifle bullet. Ed and Bobby sink Drew's body in the river since they cannot allow medical examiners to see the wound. Some time later, the men arrive at Aintry, where they explain that they suffered a canoeing accident at a falls just upriver and that their friend Drew must have drowned. Ed and Bobby believe they have their story straight—Lewis feigns having few memories of the "accident" and thereby escapes questioning—but they soon learn that fragments of their demolished wooden canoe were recovered in Aintry a full day ahead of when they claim their accident occurred. They modify their story to include an earlier accident which destroyed one canoe and fractured Lewis' leg. They claim to have piled all four men into their remaining aluminum canoe, which then also suffered an accident wherein Drew was lost and assumed drowned. The deputy sheriff who knew of the wooden canoe fragments being discovered was initially told by Bobby that the party was still traveling in two canoes (one wooden and one aluminum) when Drew drowned, just upstream from Aintry. Given the impossible chronology and the sudden recanting of the first story, the deputy grows highly suspicious of them and tells the sheriff that his brother-in-law has been missing since the weekend, believing Ed, Bobby, and Lewis have something to do with it. After a tense period of dragging the river in search of Drew's body, the sheriff lets them go with a warning not to return to Aintry.
Ed returns to city life, though changed, feeling a continuing connection with the river, which no longer exists with the dam's completion. He occasionally sees Bobby before the latter, his business failing, moves to Hawaii, but has little to do with him--"he would always look like dead weight and like screaming, and that was no good to me." Ed and his wife later buy a cabin on another "dammed lake" and Lewis, now with a permanent limp, buys a neighboring cottage. The novel ends by relapsing into the conventional patterns of city dwellers, almost as if nothing has changed, except for Ed's connection with the now-drowned river, which has made the rest of his conventional life tolerable.