In the unforgiving northern frontiers of the Roman Empire, Centurion Marcus Scaeva and Cohors III of Legio IX Hispana battle rebellious tribes, unaware that a far more advanced alien civilization, the Progenitors, is silently observing. This ancient alien race, seeking a physically superior and aggressive species for their own galactic "Project Legio," identifies the disciplined and resilient Roman soldiers as ideal candidates. While Scaeva navigates treacherous ambushes and the ambitious plans of his superiors, the Progenitors meticulously study Roman tactics, physiology, and societal structures, preparing for an intervention that will change the course of human history.

Following a daring patrol deep into the hostile Silva Umbra, Scaeva and his men are abducted by the Progenitors. Transported to a hidden alien facility light-years from Earth, they awaken to a new, terrifying reality as captives and subjects of advanced alien study. Stripped of their weapons and freedom, the Romans must draw upon their iron discipline and indomitable spirit to survive. As the Progenitors begin their systematic process of assessment and conditioning, Scaeva and his soldiers face their greatest challenge: to resist an enemy beyond their comprehension and fight for a future they never imagined.