Atlanta businessmen Lewis (Burt Reynolds), Ed (Jon Voight), Bobby (Ned Beatty), and Drew (Ronny Cox) decide to canoe down the fictional Cahulawassee River in the remote Georgia wilderness, expecting to have fun and see the glory of nature before the river valley is flooded over by the upcoming construction of a dam and lake. Lewis, an experienced outdoorsman, is the de facto leader. Ed is also a veteran of several trips but lacks Lewis' machismo. Bobby and Drew are novices.
From the start, it is clear the four are aliens in this unknown locale. The redneck locals are crude and unimpressed with the presence of outsiders, and the film implies that some of them are inbred. Drew briefly connects with a local mentally-challenged, inbred, banjo-playing boy by joining him in an impromptu bluegrass jam (the 'Dueling Banjos'). But when the song ends, the boy turns away without saying anything, refusing the impressed Drew's handshake. The four "city boys", as they are called by one of the unfriendly locals, exhibit a slightly condescending attitude toward the locals; Bobby, in particular, is patronizing toward them. Nevertheless, Lewis manages to negotiate with a pair of local mechanics (by bribing them) to drive their Bronco vehicles to a town downriver, where they will pick them up in three days. Afterwards, the four get into their two canoes and paddle off.
The men spend the day canoeing down the river in pairs before camping by the riverside at night. Shortly before they retire for the night, Lewis tells the others to be quiet and disappears into the dark woods to investigate a sound he heard. He returns shortly after and says that he didn't find anything. When asked whether he heard "something or someone," he tells them he doesn't know.
The next morning, Ed wakes first, and heads into the woods with his bow and arrow. He sees a deer, but cannot keep his aim straight. He fires and misses. He returns as the others are finishing breakfast and loading the canoes. Bobby and Ed get away first, and Lewis says that he and Drew will catch up.
After a while, Bobby and Ed pull off to the side to wait, and notice a pair of unkempt hillbillies emerging from the woods; one toothless (Herbert 'Cowboy Coward), and one wielding a shotgun (Bill McKinney). Bobby speculates that they have a still hidden in the woods and amicably offers to buy some, but the hillbillies immediately become angry, and are not moved by Bobby's sarcastic comment. The two hillbillies force Bobby at gunpoint to strip naked. The lead hillbilly chases after and physically harasses Bobby as he tries to escape. Bobby's ear is twisted to bring him to his hands and knees, and he is then ordered to "squeal like a pig". Ed is bound to a tree with his own belt by the toothless hillbilly, helpless as McKinney's character violently sodomizes Bobby.
As the two hillbillies now set their sights on Ed's "pretty mouth", Ed notices Lewis sneaking up behind them through the trees with an arrow drawn. Lewis shoots and kills the rapist with a single arrow through his back, as the toothless hillbilly quickly escapes into the woods. Lewis and Drew begin to argue with each other about whether to inform the authorities. While Drew wants to find a phone to call the police, Lewis argues that they would be arrested and not receive a fair trial, as he claims that the entire local hillbilly population are related to one another, and the jury would be comprised of "the dead man's friends and relatives". Likewise, Bobby does not want the incident of his sodomy to become public. Lewis tells them that since the entire area will be flooded by a lake soon, the body would never be found, and that the escaped hillbilly could not possibly risk informing the police on them, since he had participated in the incident. The men vote to side with Lewis' recommendation to bury the dead hillbilly's body and continue as though nothing had happened. During the digging, Drew is obviously agitated, and screams at each exertion.
After having buried the body, the four make a run for it downriver, cutting their trip short. Soon, disaster strikes as the canoes reach a dangerous stretch of rapids. In the lead canoe, Ed repeatedly asks Drew to don his life jacket, but an unnerved Drew ignores him without a word of explanation. As Drew and Ed reach the rapids, a faint gunshot is heard and Drew's head appears to shake, perhaps from being shot, and he falls forward into the river.
After Drew disappears into the river, Ed loses control of his canoe and both canoes collide on the rocks, spilling Lewis, Bobby, and Ed into the river. Ed and Drew's wooden canoe breaks in half while Lewis and Bobby's metal canoe remains intact. Lewis breaks his right femur on a rock and the others are washed ashore alongside him in a gorge facing a massive cliff. The badly-injured Lewis believes the toothless hillbilly shot Drew and is now stalking them.
Later that night, under cover of darkness, Ed climbs the nearby rock face in order to dispatch the suspected shooter using his bow, while Bobby stays behind to look after Lewis. Ed reaches the top and hides out until the next morning, when he sees a man standing on the cliff holding a rifle, looking down into the gorge where Lewis and Bobby are hiding. The man appears to be the hillbilly that escaped through the woods.
Ed, a champion archer who earlier lost his nerve while aiming at a deer, again freezes in spite of his clear shot. The man notices Ed and fires his rifle as Ed clumsily releases his arrow. Ed falls to the ground in a panic and accidentally stabs himself with another of his arrows. The man stumbles toward Ed and is about to kill him when he drops his rifle and collapses, revealing Ed's arrow sticking through the back of his neck (Ed had in fact hit him after all). Ed suddenly remembers that the hillbilly who tried to assault him had no front teeth, but upon initial examination, this man appears to have all his teeth, but upon closer examination, Ed discovers that his victim has a partial, movable plate for his front teeth... which are missing.
Ed lowers the body down the cliff with a rope and climbs down after it. His rope breaks and he and the dead body fall into the river. Ed swims ashore and meets with Bobby and Lewis. Ed asks more than once if Bobby is certain the dead man is the same toothless hillbilly as the one they confronted earlier. Clearly irritated and not completely sure himself, Ed snaps at Bobby and asks him to confirm the dead man's identity.
Ed and Bobby weigh the dead hillbilly down with stones and drop him into the river and then set out with the injured Lewis in the surviving canoe. A little later, they come upon Drew's grotesquely-contorted corpse. After being unable to find any definite gunshot wound on the dead Drew (save for a deep gash on his forehead which may or may not be a graze from a bullet), they also weigh it down into the river. Ed points out that they don't want the authorities examining Drew's body and possibly discovering a gunshot wound. Ed gives a short eulogy and sinks Drew into the river to ensure that he will never be found. With Lewis injured and Drew dead, Ed becomes the leader, trying to ensure their story is consistent, knowing the authorities will investigate.
After canoeing through more rapids, Ed, Bobby, and Lewis finally reach their destination, the town of Aintry, which will soon be submerged by the river and is currently being evacuated. They call the police and take the injured Lewis to the local hospital while Sheriff Bullard (played by the author James Dickie himself) arrives to investigate the incident. One of the deputies, Arthur Queen, has a missing brother-in-law (ostensibly one of the hillbillies Lewis and Ed killed) and is highly suspicious of the "city boys" version of events. Ed and Bobby visit Lewis' hospital room to make sure Lewis' version of events is consistent with theirs. While they are uncertain whether the apparently unconscious Lewis understands them, as the doctors enter, Lewis appears to awaken, gives Ed and Bobby a knowing wink, and says he remembers nothing. Ed and Bobby then spend the night at a local boarding house and have an uneventful dinner with the local tenants who tell them that they are leaving town soon because of the dam-lake project.
Another day or two later, Ed and Bobby pack up their two Bronco trucks outside the boarding house and they prepare to pick Lewis up from the hospital and drive home. Just then, Sheriff Bullard suddenly appears and asks Ed why there were four life jackets found in the canoe when only he, Lewis, and Bobby came out of the river. Stammering, Bobby suggests there may have been an extra one, then realizes his mistake. But Ed intervenes and says 'no', that Drew was not wearing his life jacket when he fell into the river and he does not know why. The sheriff also tells them that his men found the remains of one of the canoes above the last set of rapids where the guys claim that Drew drowned, plus no trace of Drew's body has been found along the river. Ed continues to feign ignorance, claiming not to know anything as well as claiming not to have seen anyone else in the area.
While the Sheriff remains suspicious, but having no evidence to arrest them (or perhaps sensing the truth of what happened) he simply tells Ed: "Don't ever do nothin' like this again. Don't ever come back up here. I don't ever wanna see you or your friends again! I'd kinda like to see this town die peaceful," to which Ed readily agrees and drives away with Bobby.
The men vow to keep their story a secret for the rest of their lives, which proves to be psychologically burdensome for Ed; in the final scene, he awakes screaming from a nightmare in which a dead man's hand (presumably that of the possibly-mistaken man shot by Ed's arrow on the top of the cliff) rises from the lake.