In the Arctic, a group of workers is excavating deep inside a frozen cave. Suddenly they find something stuck inside the ice, so they cut a big block to get it out and then a helicopter takes it to a lab. The scientists immediately send a fax to the excavation company, explaining there’s been a very important archaeological discovery and requesting to stop the drilling.
They also ask for a specialist to study whatever is inside the ice block. Sometime later, anthropologist Stanley arrives at the lab. He’s a doctor familiar with the Arctic because he’s been studying the history and culture of Eskimos.
Stanley is introduced to Doctor Diane and the other scientists, who believe they’ve found some prehistoric animal. In 1898 the Russians found a mammoth with still viable cells, so they have high hopes for their discovery. Then the group uses a laser to very carefully cut the ice block layer by layer to avoid any damage to the creature.
To their surprise the being inside isn’t an animal, it’s actually a Neanderthal. Stanley analyzes his appearance and concludes he’s a hunter who lived around twenty to forty thousand years ago. He wants to take a closer look, but Diane explains that their goal is to cut the caveman into pieces and send them to the best research centers in the world.
Afterward the scientists thaw the caveman and take off his clothes and tools to start taking tissue samples. They also give him a bypass and monitor his brain using electrodes. The tests indicate that the Neanderthal’s cells and organs are in perfect condition and Diane theorizes that ancient humans had cryoprotectants, which are special cells that prevent crystallization.
While the tests continue, the scientists are shocked to suddenly notice some light brain activity coming from the Neanderthal. They decide to try to revive the man, so they intubate him and shock him with a defibrillator until they can detect a strong heartbeat. Once the caveman is stable, they remove the intubation and the Neanderthal begins to wake up.
He is very confused by his surroundings, the noises, and the masked faces, so he immediately starts panicking. Stanley takes his mask and the caveman calms down when he sees a human face, allowing the others to put him to sleep with an injection. Afterward the group discusses the future of the caveman, already hoping for a Nobel.
The Arctic team is trying to develop a cryoprotectant to freeze cancer patients and unfreeze them when humanity finds a cure. But now that they have the Neanderthal, they think they can find a way to freeze people to achieve immortality. Stanley disapproves of the idea, pointing out that humans shouldn’t live forever and that the world is overpopulated.
His concerns are ignored and the scientists agree that they won’t tell the press for now because they don’t want the Neanderthal to be turned into a celebrity. Then Stanley says that the caveman should first be studied in a natural environment, which would teach them more about prehistoric times. After lots of begging, he convinces the team to let him have two weeks to conduct his own research.
Soon a special vivarium is set up and they put the Neanderthal inside. When he wakes up, he finds the environment familiar and begins going through his usual routine. He drinks juice from berries to deal with his thirst and chases a bird away from its nest to steal the eggs.
In the evening he starts a fire using stones and cooks his food. In the mornings he washes his body under a waterfall before he goes hunting with a spear he builds himself. He makes a bunch of noises that get the attention of a wild boar who tries to attack him, but the caveman dodges it at the last second and the boar gets stuck between rocks, allowing him to kill it.
Later while he’s fishing, he accidentally finds a hose and breaks the end, confused by the metal. After lots of sniffing and licking he starts following the hose, pulling it out from the ground until he reaches a metal door. He shakes it until he manages to open it, so the scientists rush to close a second door to keep him from entering.
The creatures from the lab start roaring as the scientists begin making lots of noises to scare the man away. Furious, the Neanderthal climbs the waterfall and finds the machine that powers it. He starts screaming as he tries to break the machine and hits the wall, so a scientist enters the vivarium and shoots him with a tranquilizer dart.
Then they put him on a stretcher to take him to an operation room, where they will take a bunch of samples as they originally planned. Stanley freaks out and reminds them that his two weeks aren’t over yet, but Diane tells him to accept things as they are. In the evening Stanley shows Diane a video showing different tribes capable of dancing while stepping on flames, but Diane says it’s just a nerve block.
Stanley points out that only explains the lack of pain, not the lack of blisters on the skin. Next he fast-forwards the video to find a scene where an indigenous man is putting needles in his friend’s face. Since there’s no blood, Stanley thinks they’re producing some chemical that protects their tissue.
He theorizes the caveman produces his own chemical as well and convinces Diane to let him study him some more. Once the Neanderthal is back in the vivarium, Stanley enters it too without carrying any weapons or objects to avoid scaring him. The wary caveman begins running around, growling and yelling while finding spots to protect himself.
Stanley is scared but still approaches him slowly until they’re face to face. After some hesitation, the caveman jumps on Stanley and gets ready to hit him with a rock, but he gets distracted by a scientist yelling from above. Since Stanley doesn’t try to defend himself, the Neanderthal drops the rock.
Then he takes a closer look at Stanley, noticing how different his body is from his own. The man puts his fingers in Stanley’s mouth, pulls at his clothes, and squeezes his groin to confirm he’s not missing certain parts. Stanley screams in pain and the Neanderthal backs off before introducing himself as “Ar-ru”, which Stanley changes into Charlie.
In return Stanley teaches him to say his name and even makes Charlie laugh. Noticing the scars on his chest Stanley asks about them and Charlie calls them “be-tah” while pointing up. The shape of the scars and the gesturing make Stanley think it means “bird”.
Charlie steals Stanley’s cross necklace and after biting it, he begins repeating some words and gesturing with his hands, but Stanley can’t understand what he’s trying to say. The next day linguist Mabel arrives and Stanley shows her the recording of his conversation with the caveman, but she’ll need more to fully understand him. Then Stanley starts spending time with Charlie and learning many words in his language for simple things like “rock”, “frog”, and “bone”.
Charlie also shares his food with him. In return Stanley teaches him easy words in English. Mabel watches everything on the cameras and is impressed by Charlie’s extensive vocabulary.
One night while they sit by the fire, Stanley starts singing a song. Charlie likes it and encourages him to keep going. While playing with the bones from his meal, Charlie makes some noises to happily sing along.
Sometime later the scientists put Charlie to sleep again to take more samples and start a new experiment. He’s put inside a special chamber at a very low temperature; hopefully this will activate the cryoprotectants and make it easier for the scientists to extract them. Stanley keeps on protesting, saying they’re killing Charlie and that he doesn’t deserve to be treated like a lab rat.
Charlie’s heartbeat starts slowing down and the scientists have no choice but to get him out. Afterward in the vivarium, Charlie keeps yelling in anger at Stanley because he isn’t happy about all the experimenting and new scars. Suddenly he jumps and pretends to attack Stanley, hitting the ground with his spear before poking his own chest.
He’s asking Stanley to help him self-delete but Stanley refuses to do it, and after some struggle he leaves the vivarium. Later in private Stanley asks Diane for help. All the studies are slowly alienating Charlie and causing them to lose contact with him.
Stanley wants Diane to go to the vivarium with him because perhaps the presence of a woman will help him calm down. When they enter the vivarium, Charlie’s hands are wounded and he’s using rock powder to draw the bird from his scars on the ground. Diane is very nervous and gets scared when Charlie immediately runs to her for a better look.
After some sniffing he shares his food with Diane, and she eats it not to offend him even though it’s made of bugs. Then Charlie keeps on touching, being friendly at first but soon trying to look at her groin too. Stanley immediately stops him and the men yell at each other, so Stanley has to use the Neanderthal word for “mine” to make Charlie back off.
Then Charlie brings some objects from his cave to exchange them for Diane, so Stanley has to turn him down. Looking at Diane puts Charlie in a sad mood and he repeats the same words over and over. When Mabel analyzes this, she thinks she’s found Charlie’s word for "woman", but there’s something else she can’t quite understand.
Stanley and Diane make Charlie explain the words and he does some drawings on the ground, revealing he used to have a wife and kids. He starts running around repeating those words and Diane realizes he’s asking where his family is. The moment is interrupted when a helicopter appears above the facility.
Since the ceiling is made of glass, Charlie sees it and freaks out. He starts climbing up the rocks while repeating “be-tah”, his word for bird. The scientists immediately tell the helicopter to leave, but Charlie won’t calm down.
He climbs to the top of the vivarium and keeps screaming “be-tah” at the ceiling. Sometime later Stanley visits a tribe of Eskimos and shares the words he heard from Charlie, which the the group recognizes. Stanley rushes back to the lab to share his discovery with Diane: “be-tah” refers to Sedna, the legend of a large bird that acts as God’s messenger and carries people to heaven or hell depending on their behavior.
He theorizes that Charlie’s tribe went extinct when the magnetic pole flipped and killed all the animals in the area. Charlie probably felt it was a punishment from God and went on a dream walk, meaning he went out to find the bird and offer himself in exchange for the return of the animals. Charlie now thinks the helicopter is Sedna, so there’s no way they’ll calm him down.
Thirty–four hours later, Charlie is wearing all his clothes, tools, and various trinkets Stanley gave him the past few days. He also uses crushed berries as paint to decorate his skin. Charlie never stops chanting, obviously eagerly waiting for the return of his God.
Whenever Stanley enters the Vivarium, Charlie hides until he’s gone, refusing to communicate. One afternoon, Charlie takes the stairs and after watching Stanley so many times, he knows he must press the button. He enters the lab and carefully sneaks around, making sure the scientist won’t see him.
When he reaches an empty corridor, he finds a mirror and starts growling at his own reflection. Thinking the other man is challenging him, Charlie breaks the mirror with his spear. Then he hears a voice in the speakers and runs through the corridor in fear until he reaches the room with all the lab animals.
As he checks out the bird, the bear growls so Charlie keeps on moving. As he tries to take the next corridor, he crashes against the glass doors. He quickly tries again and the doors open automatically, allowing him to enter another room.
At that moment he gets scared when he hears a vehicle moving, so he hides behind a copy machine. However Charlie accidentally activates it and gets afraid again, so he runs. After getting startled by the elevator, he enters a lab and breaks a jar with acid, which slightly burns his hand.
At that moment the scientists turn off the lights, causing Charlie to panic. He starts climbing on the furniture, only to keep slipping and breaking equipment. A scientist hears the noise and comes inside as he turns the lights back on.
Charlie is startled again and immediately attacks, killing the man with his spear. Then Charlie begins running once more, going crazy when he suddenly hears the alarm. He crosses a random door and finds a tunnel that gets colder the further he goes.
When he finds the fire exit, he quickly goes through it and finally makes it outside. He sees the helicopter and chases after it, so the pilot hits him with the skis to knock him out. The scientists finally show up and rescue Charlie, bringing him back inside.
Moments later they look at the x-rays, which confirm Charlie has no broken bones or head injuries. The scientists are furious with Stanley, blaming his research for the death of their colleague. They announce the experiments are over and Charlie will be transferred to another facility.
Stanley tries to explain Charlie will die if he’s kept in a cage, but his concerns are ignored. Refusing to give up, he secretly makes a deal with the guy guarding the door. Soon Stanley enters the vivarium with a bunch of furs.
Once Charlie is properly protected from the cold, Stanley takes him out for a walk. The scientists realize they’re missing and go looking for them by following their footprints in the snow. As the duo keeps going, the ice shelves start falling and a crevasse suddenly opens in the ground, separating Stanley from Charlie.
When the helicopter finally finds them, Charlie gets excited and yells “be-tah” before jumping. He manages to hold on to the chopper’s skis and happily chants during the flight. One of the pilots tries reaching for him, so Charlie lets go of the skis and falls, gliding for a few miles in a rather good mood before he meets his end.
