Plot

In the Old Stone Age, a disparate gang of early humans band together in search of a new land. But when they suspect a malevolent, mystical being is hunting them down, the clan are forced to confront a danger they never envisioned.

In Upper Paleolithic Europe, 45,000 years ago, six humans arrived on the shores of unknown land in search of a better life: group leader Adem, his pregnant mate Ave, his younger brother Geirr, and his 11-year-old son Heron. Beyah, a young stray female, and Odal, an elder male, are also with them.

As they journey toward the mountains, searching for a cave to settle, the group encounters a mysterious creature, and Heron is abducted at night. The group is forced to follow Adem when he insists on pursuing it into the woods. They become lost in the forest, and Adem attempts to track the creature alone as darkness falls. He is attacked and is brought back to the group's camp by Geirr with mortal wounds. Beyah mercy kills Adem, and at her suggestion, all but Geirr cannibalize his body to avoid starving.

Geirr unsuccessfully attempts to lead the group through the woods. Odal, believing that they are being stalked by a demon, blames Beyah's menstrual cycle and attempts to sacrifice her with Ave's help. During the scuffle, Geirr is knocked unconscious, and Beyah escapes; Odal instead offers a replacement by stabbing Ave, but she manages to break his ankle in the process. With the forest lit by aurora borealis, Odal watches as the creature takes Ave's body. At dawn, Beyah finds Geirr, and they witness the creature killing Odal and ambush it, but it escapes. They discovered it was a female Neanderthal wearing a mask. They trail her to the mountain and find a cave where Beyah believes the Neanderthal lives.

After Beyah enters the cave, she is attacked by a male Neanderthal who pins her to the ground, attempting to strangle her. She feigns death and then stabs him, compelling him to roll off of her. Beyah retrieves her wooden spear and mortally wounds the Neanderthal. Then she continues into the cave, where she discovers an unharmed Heron and is confronted by the Neanderthal female. Geirr, hearing Beyah's screams, finishes off the dying male Neanderthal and continues through the cave to find Beyah. The female Neanderthal then attacks Geirr with a stone ax and kills him. The female Neanderthal threatens Beyah, who sets fur bedding on fire and escapes with Heron through a small vertical opening in the cave. The female Neanderthal attempts to escape but becomes stuck at the top of the opening, but while Heron tries to help her, Beyah kills her with a rock. She tells Heron the Neanderthals are monsters who killed the group, while he says, "They are like us," implying that they abducted him to feed and protect him.

The film ends with Beyah detailing their story; she seems to regret killing the Neanderthals, remarking they were just people as terrified of the unknown as her group. Heron shows Beyah Ave's body laid out in the cave, which had been prepared for a funeral by the Neanderthals. The two bury the dead, and Beyah, now claiming the cave, remarks, "She and the boy learned to survive, the first of a new people." Her last words in answer to Heron's question: "What do we do now?" are "We try again," presumably with the lessons that they have learned. (thanks to Wikipedia)