In the year 43,000, B.C., a starving, ragtag group of six prehistoric humans has reached the shores of a new world searching for food and a new place to live. Adem is the leader of the group and has been joined by his pregnant mate, Ave, his younger brother, Geirr, and his 11-year-old son, Heron. Also with them are Beyah, a young female ‘stray’ with a low ranking in the group, and Odal, an elder who was allowed to accompany the group because of his wisdom. Heron asks Beyah for a story, but after her refusal, Odal relates how the boy’s father has taken them on an adventure based on childhood stories in order to find a new life.
As they journey towards the mountains to find a new cave, the group is ambushed in the night by a mysterious creature, and Heron is abducted. The group wait until morning to track the creature and the young boy, and are forced to follow Adem when he insists on pursuing the creature into the woods where the trail leads. The group becomes lost in the forest, and as darkness falls, Adem hears mysterious noises in the wood and runs in pursuit while the others hold back. Adem is attacked, and is brought back to the group’s camp by Geirr with mortal wounds. Geirr is unable to put Adem out of his misery, so Beyah steps forwards to end his suffering and then suggests the group cannibalize the body to avoid starving. Geirr is unable to consume his brother’s flesh and buries it in the ground, but the others in the group partake. Geirr, now assuming leadership of the group, attempts to lead the group through the woods and toward the mountain where he hopes that they will find a cave, but only manages to lead them in circles. Odal, believing that they are being stalked by a demon, blames the catastrophe on Beyah and attempts to sacrifice her to it with Ave and Geirr, but she escapes, knocking out Geirr in the process. Odal, in turn, offers a replacement sacrifice to the demon by stabbing Ave with a tree branch, but she breaks his ankle with her last breath. During a night of aurora borealis, a crippled Odal watches as the creature takes Ave’s body. The following dawn, Beyah finds Geirr and they decide to go towards the mountain. They then witness the creature killing Odal and ambush it, only to discover it is a female Neanderthal wearing a mask. They trail her to the mountain and find a cave, where Beyah believes the Neanderthal has taken shelter.
As Beyah enters the cave, she is attacked by a male Neanderthal and nearly killed. She feigns death, but then attacks and mortally wounds the man. She continues into the cave where she discovers an unharmed Heron in a bed of furs, whereupon she is confronted by the Neanderthal female wielding a rock hammer. In the meantime, Geirr, hearing Beyah’s scream, tries to follow her and comes across where the male Neanderthal lays dying. He finishes him off with his spear and continues through the cave to find Beyah, finally reaching her at the cave, whereupon the female Neanderthal attacks Geirr and kills him. The female then threatens Beyah, who promptly sets the furs on fire and escapes with Heron through a small opening in the cave that serves as a chimney. As the female Neanderthal attempts to escape through the hole, Heron tries to help her, but Beyah instead smashes her head in with a rock. She tells Heron the Neanderthals are monsters who killed his father, while he says they abducted him in order to feed and protect him, knowing he would starve if he remained with the group.
The film ends with Beyah’s story to Heron, detailing her regret of killing the Neanderthals, and remarking that they were just fearful human beings, as terrified of the unknown as Beyah and her group were. Heron shows Beyah Ave’s body laid out in honor, covered in ritual paint and funeral clothing as prepared by the Neanderthals. The two proceed to bury Ave, Geirr and the two Neanderthals. Beyah, now claiming the Neanderthals’ cave as her and Heron’s own, remarks that they will be the first of a new people, and that they will try again to survive with the lessons that they have learned.