"I'm a Behemoth" is a fantasy anime series set in a world where humans coexist with mythical creatures and adventurers seek fame and fortune through dangerous quests. The protagonist, a skilled adventurer, is unexpectedly transformed into a powerful behemoth, a legendary beast known for its immense strength and ferocity. This transformation occurs during a high-stakes mission, leaving the protagonist to navigate a new existence as a creature feared by many.

The narrative explores the protagonist's struggle to adapt to their new form while retaining their human consciousness. Despite their monstrous appearance, they retain their sense of morality and continue to protect those in need, often using their newfound abilities to aid others. The series delves into themes of identity, acceptance, and the duality of strength and vulnerability, as the protagonist grapples with their altered state and the reactions of those around them.

Key characters include the protagonist's loyal companions, who remain by their side despite the transformation, showcasing themes of friendship and loyalty. Among them is a skilled healer who provides emotional support and a pragmatic warrior who helps strategize their battles. Antagonists include rival adventurers and powerful entities who seek to exploit the protagonist's unique abilities for their own gain. The series balances action-packed sequences with moments of introspection, as the protagonist confronts both external threats and internal conflicts.

The plot progresses through a series of quests and encounters, each revealing more about the world's lore and the protagonist's evolving role within it. As the story unfolds, the protagonist uncovers deeper mysteries about the origins of their transformation and the true nature of the behemoth, leading to a climactic confrontation that tests their resolve and challenges their understanding of strength and humanity. The series concludes with a resolution that ties together the protagonist's journey, leaving room for potential future developments.