"Leviathan" is an anime series set in a dystopian future where humanity faces extinction due to the emergence of colossal, monstrous creatures known as Leviathans. These beings, seemingly invincible, have driven humans to the brink of survival, forcing them to live in isolated, fortified cities. The story centers on a group of elite soldiers, known as the Leviathan Hunters, who are tasked with combating these creatures to protect the remnants of human civilization.

The protagonist, Kael, is a skilled but emotionally scarred soldier who lost his family to a Leviathan attack. His journey begins when he joins the Hunters, driven by a desire for vengeance and a determination to uncover the truth behind the Leviathans' origins. Alongside Kael is Lira, a brilliant scientist whose research into the creatures' biology becomes crucial to understanding their weaknesses. The group also includes Ryn, a seasoned veteran with a pragmatic approach to survival, and Eris, a mysterious rogue operative with her own hidden agenda.

As the Hunters embark on missions to confront the Leviathans, they uncover a deeper conspiracy involving human experimentation and the possibility that the creatures were not a natural phenomenon but a man-made catastrophe. The narrative explores themes of survival, morality, and the consequences of humanity's hubris. Tensions rise within the group as they grapple with their own motivations and the ethical dilemmas posed by their discoveries.

The series balances intense action sequences with moments of introspection, delving into the psychological toll of living in a world dominated by fear and loss. The Leviathans themselves are depicted as both terrifying and enigmatic, with their origins and purpose gradually revealed as the story progresses. The Hunters' battles against these creatures are not only physical but also symbolic, representing humanity's struggle to reclaim its place in a world that has turned against it.

"Leviathan" concludes with a climactic confrontation that forces the characters to confront their pasts and make sacrifices for the future. The ending leaves room for interpretation, with lingering questions about the true nature of the Leviathans and the possibility of coexistence or redemption. The series is a blend of science fiction, action, and drama, offering a thought-provoking exploration of humanity's resilience in the face of overwhelming adversity.