Twenty-eight-year-old Rebecca (Beck) Shelton is not exactly looking forward to the survival weekend her deputy sheriff husband, Reed, has planned for them. Reed hopes the wilderness training in the dense woods of Idaho will help his shy, stuttering wife overcome her fears.
Randy Thompson, a survival expert, is supposed to help them prepare a camp and look for food. Their friends Dr. Michael (Cap) Capella and his wife, Sing, will rendezvous with them in the morning. Reed and Beck reach the cabin where they'd planned to meet Randy Thompson, but it has been torn apart. Randy is nowhere to be found. Reed calms Beck's fears, and the two set up camp away from the cabin, but still within sight of it.
Meanwhile, when Cap and Sing arrive at the local lodge, the manager, Arlen Peak, tells them that strange things have been happening. Just a few hours earlier, a herd of elk ran through as if fleeing something. He warns the couple to be careful when they make their way into the woods the following day.
That night, the sounds of large animals wakens Reed and Beck. At first they believe the sounds are from a bear, but soon a woman's shrieking gets their attention. They realize that three large animals are circling them, and the shrieking might be a distant beast. Beck and Reed flee into the forest as they try to figure out what is happening. An overwhelmingly foul stench accompanies the beasts, so the couple knows when they are near. When Beck tries to climb a tree for safety, she finds the torn body of Randy Thompson. They continue to look for a way to escape; Beck inadvertently careens off a waterfall as Reed watches helplessly. A large creature scoops Beck from the river and carries her into the forest.
The following morning, Cap and Sing find Reed but can't locate any sign of Beck or Randy Thompson. Local law enforcement, volunteers and hunters set up a base camp at the lodge in order to search for Beck and kill the bear they believe has carried her off. Reed tries to convince Sheriff Mills and the others that the beast he saw was larger than a bear and walked upright, like a human, but they think he is in shock.
Beck wakes up in the company of a large, female, ape-like creature. The foul smelling animal feeds her a mouthful of berries. When she tries to leave, Beck realizes that her ankle is badly sprained, which severely handicaps her ability to outrun the beast. She is its hostage, until Reed finds her or she can figure out a way to escape.
The volunteers are broken into two groups. One examines the cabin and looks for clues. The other group searches for Beck. The search party finds a few odd tracks in the mud but has difficulty following the trail. A rescue dog acts strangely and refuses to lead its handler further into the woods. The random tracks indicate a large animal with a strange gait, but the men still think it must be a bear. They locate Beck's backpack, but no other evidence of her, except a strange lock of hair. Reed takes the pack back to the lodge and discovers that everything is still inside it. Cap explains that his and Reed's friend Pete, the search and rescue manager, has found more prints and hairs in the woods that might belong to the beast.
Cap hints that Pete may believe Reed's story, but he won't make it official until he has more proof. Reed begs Cap to take some of the hair samples to the university he used to work at in order to run tests to see what kind of animal they're dealing with. Cap confesses he can't go back without more evidence and hints that he did not willingly leave his job there.
Beck's captor takes her to a river where Beck gratefully drinks and tries to clean herself. As they wade in the water, two more of the creatures arrive, a female and her son. These two creatures do not welcome Beck, but her captor protects her from their attacks. Her captor continues to dote on Beck after the others leave, pulling Beck close during the night in order to keep her warm.
Several miles away, a young married couple set up a campsite in the woods and begin to cook their dinner over an open fire. They hear the same shrieking woman that Reed and Beck heard the night before. When they investigate, a large beast attacks the husband. The wife hits the animal with a hot frying pan, and the couple manages to escape.
When the investigators question the couple, Jimmy, another deputy sheriff, believes the couple are the victim of a bear attack, but Reed is convinced they saw the same humanoid animal he did. A local merchant who heard about the animal after the couple stopped at his store arrives at the campsite. The merchant is a Native American. He and Sing, who is also Indian, discuss the legends of the wild men, or tsiatko. Other tribes called them sess-ketch; the white men call them Sasquatch.
The merchant says the tsiatko have laid claim to the woods. They will attack and kill any who wander in it. He runs through the area declaring to the tsiatko that he and his family will never set foot on this ground again. Jimmy believes the man is crazy and gathers some men to hunt for the bear in the morning. Cap, Reed, Sing and Pete continue to search the area and find the animal's dung. They collect a sample and convince Cap to take it and the hair samples to the university. Reed starts thinking more like a police officer and asks the sheriff if he might use the computer in his office to look for a pattern in the attacks. He discovers that a few days earlier, a logger had been killed in the same area. He and Sing investigate.
Beck is "introduced" to the leader of the group of Sasquatch, a large male she decides to call Jacob. Her captor reminds her of Rachel, because she doesn't have a child. The other female she names Leah. The juvenile male she names Reuben. Jacob has brought the carcass of a deer to eat and reluctantly shares some with Rachel and Beck. The following morning, the Sasquatch are frightened away by something. Beck suspects it may be people looking for her, but before she can call out, Rachel carries her away from the intruders.
An analysis of the feces and hair samples that Cap has brought is inconclusive. The lab technician believes it is because the samples were contaminated. The best he can tell Cap is that the feces came from some kind of primate.
Reed and Sing investigate the logger's death. Although originally ruled an accident, Reed and Sing conclude that the same beast that attacked Randy killed the man. The body was moved after death and covered with logs, as if to hide the manner of the man's death. They realize that something dangerous is in the forest and that someone is trying to cover up the evidence. After examining the pictures he originally took of Randy Thompson's cabin, and then looking at pictures taken later, Reed notices that there are more boot prints entering the cabin and that a shovel is missing. He and Sing share the new evidence with Pete, the game warden.
Beck tries to adapt to her role as Rachel's child. The Sasquatch is quite maternal and makes sure that Beck eats everything she is given, including raw deer meat and plants. Beck is overjoyed when Jacob returns with real fruit — apples and pears.
Reuben steals Beck's apple and beats her when she tries to take it back. Beck must be satisfied with the cattails Rachel offers her. Later that night, as they travel through the woods, a bear attacks Beck, and it tears off a part of her jacket. Rachel fights the animal and scares it away. Jacob leads his pack to where he found the fruit. Beck realizes the fruit is some kind of lure. The area around it has been tilled so that any animal taking the fruit will leave prints. She quickly writes out Reed's phone number as a clue for anyone who might be tracking her. Rachel carries her off before she is able to finish her message, but Beck hopes it will be enough.
Cap makes contact with an old friend, Dr. Emile Baumgartner. Baumgartner fears what the college will do to him if he's seen with Cap, so the two drive to his home. The former colleagues discuss their differences in evolution theory as it relates to the weird DNA in Cap's feces sample. Although restricted legally from telling Cap anything definitive, Baumgartner alludes to the possibility that another scientist from the college, Dr. Burkhardt, may have been pressured by financial backers to find the "missing link" between chimpanzees and human beings.
Baumgartner suggests that Burkhardt may have tried to virally implant human DNA into chimps. This kind of mutation is imprecise at best, and scientists have no idea what kind of mutations will happen because of it. Cap returns in secret to the campus lab to see if his sample contains both primate and human DNA.
The young man who set out the fruit for the Sasquatch is ecstatic when he sees their footprints in the morning. When he notices the numbers scratched into the dirt, he calls his friend Arlen Peak, the owner of the lodge where the investigators are staying. Arlen brings pictures of the site to Reed and the sheriffs. The local deputy sheriff is dubious of the photo, but Reed knows that Beck is alive and leaving him clues.
Beck spends the morning bonding with Rachel by brushing the beast's fur. While Beck's attention is diverted, Reuben steals Beck's leather jacket and the toilet paper tucked inside its pocket. Beck is devastated at losing her one remaining reminder of her humanity. Reuben runs into the woods, tearing the jacket apart and leaving trails of toilet paper amid the trees.
Reed and others strap individual GPS trackers to their arms and start the search again for Beck. They hear Reuben in the woods and find his trail of toilet paper. Over his walkie-talkie, Sheriff Mills tells the others that he's spotted something big walking upright like a man. Before they get additional information, his transmission stops. In the woods, the men hear several screams, including that of a woman. Reed recognizes it as the same yelling he heard the night Beck disappeared. Reed, Pete and the others find Sheriff Mills' torn body. He's been killed in the same way as Randy Thompson and the logger.
The men continue their search as the Sasquatch hide in a thicket of trees. Rachel forces Beck to hide as well. As Reed approaches the thicket, Beck is terrified Jacob will attack and kill him. She remains silent as her husband searches for her, willing him to hear her thoughts and leave the area so he'll be safe. Reed finds Beck's torn and bloody jacket. Again, Beck wills him to hear that she is safe and not dead. Reed leaves the area, devastated at the evidence of his wife's death.
Cap confronts the dean of the College of Sciences with his theory of Dr. Burkhardt's experimentation with chimpanzees. While in the dean's office, Cap steals his keys and uses them to get into Burkhardt's office. It's obvious the man has cleared out, although Cap finds the remains of a cage that had been chewed through and nearly destroyed. The dean calls security, and Cap is escorted off the campus.
Beck makes friends with Leah, the alpha female, by brushing her fur. Beck discovers that a large animal has bitten her; Leah has a wound similar to the one Beck had found on Rachel. Certain that Jacob wouldn't have bitten Leah, Beck wonders what beast would have dared to attack both of them. Reuben tries to remove Beck from his mother, but both Beck and Leah stand up to him. Reuben sulks off into the woods. For the first time in a while, Beck feels empowered and unafraid.
Reed convinces Sing and Pete that Beck may still be alive. They set out more fruit as bait for the Sasquatch, this time including a GPS tracking device they hope Beck will find and turn on. Jimmy launches a hunt for what he still considers to be a bear. Everyone is warned to be extremely cautious and call for backup if anything is spotted.
Jacob returns to the Sasquatch group with fruit. Beck waits her turn to take a piece. As she reaches for an apple, she spots the GPS next to a pear. Before she can grab it, Leah examines it, then Rachel. Beck is beside herself when Rachel bites it before putting it down. As soon as Reuben realizes Beck wants the device, he takes it. Beck fights for it and is severely beaten for her effort. A distant gunshot causes the beasts to flee.
Jimmy and his crew have bagged a giant bear. When they cut it open, they find a piece of Beck's jacket. The hunters, convinced that this is the animal that killed Beck and the others, call off the search for her remains. Sing asks for a piece of the bear meat before they leave the area. She, Pete and Reed burn the meat in an Indian ceremony honoring the dead.
Beck tries several different ploys to get the GPS away from Reuben, none of them successful. Finally she humiliates herself by stuffing her clothes with leaves to make herself appear bigger and then covering her body with mud and dung to change her appearance. The other Sasquatch are confused by her show of aggression. She closes quickly on Reuben, hitting him with a tree branch. Before Beck can grab the GPS, Leah charges to protect her offspring. This spurs Rachel to enter the fray. The two female Sasquatch battle. Rachel finally overpowers Leah. Beck summons the last of her strength to face Reuben again and is able to grab the GPS and turn it on before Rachel carries her away.
Back at lodge, Sing is packing her computers and gear. A sudden blip appears on her laptop. She calls to Reed. It is the GPS they'd hoped Beck would find. It's been turned on, and whoever has it is moving quickly through the forest. Reed and Sing gather the few searchers still in the area, including three men — Max, Sam and Steve — who had assisted Jimmy in his hunt for the bear. They strap on the remaining GPS devices and set out to find Beck. All of them are wary when the same shrieking is heard that accompanied earlier fatal attacks.
As the others search for Beck, Cap looks for evidence as to what kind of animal is actually in the woods. He breaks in Dr. Burkhardt's cabin, which happens to be in the same vicinity where the logger was killed. Cap finds jars with the remains of unborn chimpanzees. Then, from a metal outbuilding, he hears the wailing of a woman. When he investigates, he finds mutated chimps in cages. One is smaller than him in height but much more muscular. Deranged, it fights against its cage and makes a sound like a woman in pain. Cap discovers that something has torn a gaping hole in the back of the building. The dean arrives with armed guards. Cap tries to get the dean to understand that Burkhardt's mutations don't prove anything about evolution and that the doctor fled the area because one of his creatures has escaped. Cap leaves to warn his friends about what exactly is in the woods.
As Reed tracks Beck, a conspiracy is uncovered. Max, one of his fellow searchers, is actually Dr. Burkhardt. Sam and Steve work for the doctor and are systematically killing off any of Burkhardt's monsters that they find. Steve returns to the lodge and shoots Sing to foil the search, but Sing survives. She stabs Steve, and then re-establishes contact through the GPS with Reed. Reed kills his stalker, Sam, and then continues to search for Beck.
The Sasquatch flee from the wailing beast. Beck recognizes the place where she and Reed had camped. Rachel stops running, puts Beck down and explores the area until she finds the mutilated body of another Sasquatch. Beck realizes that the animal must have been Rachel's child. Beck is the same size and has the same hair color as Rachel's dead offspring. As Rachel howls in grief, the monster in the woods answers. Jacob forces Rachel to leave, but this time, Beck is not taken with them.
Beck turns on her GPS and calls out for help. Instead of aid, she catches the attention of the wailing beast. Beck flees, but the monster pursues her into a clearing. Before it can tear her apart, Dr. Burkhardt shoots it. The exhausted doctor confesses his crimes to Beck. She realizes he must kill her in order to keep his secret. As he attacks her, the Sasquatch return. Jacob flings the doctor away. Rachel embraces Beck and prepares to take her, but Reed enters the clearing. Through a series of grunts to Rachel and commands to Reed, Beck manages to make the Sasquatch understand that Reed is not a threat. As Reed bows in submission to Jacob, Beck limps toward him and the two embrace. Although obviously saddened, Rachel leaves with the other Sasquatch. Dr. Burkhardt staggers out of the trees, leveling his gun at Reed and Beck. Reed convinces the doctor not to kill them as they all hear the sound of an approaching rescue helicopter.
Dr. Burkhardt and the dean are imprisoned for their crimes. Cap lets his friends know that the university may restore his job as a research scientist. Jimmy thinks the Sasquatch were all Burkhardt's mutations until Cap reminds him that one of the samples of hair showed clean primate DNA. Beck, who no longer stutters, returns to the forest and calls out a farewell to her Sasquatch family. From the distance, she hears a faint answer.