In Upper Paleolithic Europe, approximately 45,000 years ago, a group of six individuals arrives on the shores of an unfamiliar land, searching for a brighter future. This group consists of the steadfast leader Adem, his pregnant mate Avé, his younger brother Geirr, and his inquisitive 11-year-old son Heron. They are accompanied by a young stray female named Beyah and an older male known as Odal.
As the group ventures toward the mountains in pursuit of a cave for shelter, they encounter a mysterious creature that haunts the dark woods. Tragedy strikes when Heron is abducted during the night by this creature, prompting Adem to insist on tracking it down despite the unease of the others. The situation takes a grim turn when they become lost in the dense forest, leading Adem to embark on the hunt for the creature alone. When darkness descends, he is attacked and returns to camp gravely injured. In a moment of mercy, Beyah ends Adem's suffering, and in a desperate act for survival, the remaining group members, except for Geirr, resort to cannibalism to stave off starvation.
Geirr tries to lead the group, but things only worsen when Odal, convinced they are being pursued by a demon, blames Beyah's menstrual cycle for their misfortunes and suggests a ritual sacrifice involving her, with Avé unwillingly assisting. The ensuing struggle leaves Geirr unconscious while Beyah manages to flee. In a fit of rage, Odal strikes Avé, injuring her before she retaliates and breaks his ankle. Under the celestial glow of the aurora borealis, Odal watches helplessly as the dark creature claims Avé’s lifeless body.
At dawn, Beyah finds Geirr, and together they witness the creature, which turns out to be a female Neanderthal, brutally killing Odal. They try to ambush this being, only to find that it escapes. Following the creature, they discover a cave believed to be its home. When Beyah enters, she is confronted by a male Neanderthal who attempts to suffocate her, but she cleverly feigns death and eventually manages to mortally wound him.
Venturing further into the cave, Beyah is relieved to find Heron unharmed, but the female Neanderthal attacks Geirr, ultimately killing him. As the feral female threatens Beyah, she ignites fur bedding to create a diversion, allowing her and Heron to escape through a narrow opening. However, the female Neanderthal becomes trapped while trying to follow. Though Heron attempts to assist, Beyah makes the fateful decision to kill her with a rock after expressing that the Neanderthals were monsters responsible for their pain, while Heron argues, "They are like us," indicating his belief that they sought to protect him.
In the film's poignant conclusion, Beyah reflects on their harrowing experiences, admitting her regret over the deaths of the Neanderthals, recognizing that they were simply people grappling with their own fears of the unknown. As Heron reveals Avé’s body, meticulously prepared for a funeral by the Neanderthals, they decide to bury their dead. Claiming the cave as their new home, Beyah states, "She and the boy learned to survive, the first of a new people." When Heron asks what their next steps will be, she confidently replies, "We try again," suggesting they will move forward, applying the lessons they have learned amidst their tragedies.
