Alex, a pragmatic geology student, and Sarah, a budding archaeologist, stumble upon an unnaturally deep and mysterious sinkhole during a research trip in a remote mountain range. A sudden tremor sends them plummeting into the darkness. Instead of a mere underground cave system, they emerge into a lush, untamed world teeming with prehistoric life, seemingly untouched by human history. They quickly realize they have been sent back ten thousand years into Earth's distant past. Stranded and disoriented, they must rely on each other and their wits to survive. They encounter tribes of ancient humans – hunter-gatherers living a harsh but communal existence. Through observation and interaction, they learn about their simple clothing made from hides and furs, their diet of hunted game and gathered plants, their basic tools, and their strong tribal bonds. They face encounters with terrifying megafauna thought long extinct. As they navigate this perilous world, they search for a way back to their own time, perhaps connected to the very anomaly that brought them there. Their journey tests their courage, resilience, and their bond, ultimately leading them back to the mysterious chasm for a chance to return home.