A group of 22nd-century outcasts travels through a one-way time portal to the Pliocene Epoch, only to discover they have arrived in the middle of a brutal conflict between two extraterrestrial races. The protagonists, seeking a fresh start in the prehistoric past, find their hopes of a peaceful existence shattered by the presence of the Tanu and the Firvulag. These alien factions possess advanced technology and psychic abilities, forcing the human arrivals to navigate a dangerous political and physical landscape. The narrative employs a multi-perspective approach to track the disparate groups as they adapt to the harsh realities of a world that is far from the idyllic sanctuary they expected.

This work is widely regarded as a foundational entry in the subgenre of science fiction that blends prehistoric settings with advanced extraterrestrial technology. Readers often highlight the intricate world-building and the complex social structures of the alien races as the primary strengths of the narrative. The pacing is frequently noted for its deliberate development, which allows for a deep immersion into the unique environment of the Pliocene. The series is often cited for its ambitious scope and its ability to maintain a consistent tone across a sprawling, multi-volume arc.