"Eien no Filena" is a fantasy anime series set in a medieval-inspired world where kingdoms are locked in a perpetual cycle of war and conquest. The story centers on Filena, a young princess of the Kingdom of Satera, who is forced to disguise herself as a man to survive after her kingdom is overthrown by the neighboring Kingdom of Medallia. Raised as a warrior from a young age, Filena adopts the identity of "Phil" and becomes a gladiator in Medallia's arena, where she gains fame for her combat skills and strategic mind.

As Filena navigates the brutal world of gladiatorial combat, she forms alliances with other fighters, including the kind-hearted and loyal Kain and the enigmatic warrior Delos. Together, they uncover the political machinations behind the wars and the oppressive rule of Medallia. Filena's ultimate goal is to reclaim her homeland and restore peace, but her journey is fraught with challenges, including betrayal, loss, and the constant threat of her true identity being revealed.

The narrative explores themes of identity, resilience, and the cost of war, as Filena struggles to balance her personal vendetta with her desire to protect those she cares about. The series also delves into the complexities of power and loyalty, as characters grapple with their roles in a world dominated by conflict. Filena's growth from a sheltered princess to a determined leader forms the emotional core of the story, while the political intrigue and action-driven plot keep the narrative engaging.

The supporting cast includes key figures such as Queen Lana of Medallia, whose ruthless ambition drives much of the conflict, and the noble warrior Delos, whose past ties to Filena add layers to their relationship. The series blends personal drama with larger-scale political and military struggles, creating a multifaceted story that balances character development with high-stakes action.