"Grimoire of Zero" is a fantasy anime set in a medieval world where witches are persecuted and hunted by the Church. The narrative centers on a mercenary known only as "The Beastfallen," a human cursed with the appearance and abilities of a beast. He is hired by a witch named Zero to protect her on a journey to recover a powerful magical tome called the Grimoire of Zero, which holds the potential to alter the world. The Grimoire is said to contain the ultimate magic, capable of granting immense power to its possessor.

Zero, a cheerful and enigmatic witch, seeks the Grimoire not for personal gain but to prevent it from falling into the wrong hands. Alongside her is Albus, a young boy who serves as her apprentice and harbors his own secrets. The trio travels across a war-torn land, facing numerous adversaries, including other witches, religious zealots, and rival mercenaries, all of whom seek the Grimoire for their own purposes.

The story explores themes of prejudice, identity, and redemption, particularly through the character of The Beastfallen. As a Beastfallen, he is ostracized by society and struggles with his dual nature, torn between his human emotions and his beastly instincts. His growing bond with Zero and Albus challenges his self-loathing and forces him to confront his past. Zero, on the other hand, is driven by a sense of responsibility and a desire to protect the world from the Grimoire's destructive potential, despite her own mysterious origins.

The plot unfolds through a series of episodic adventures, each revealing more about the characters and the world they inhabit. The central conflict revolves around the race to secure the Grimoire before it can be misused, culminating in a confrontation with the true antagonist, who seeks to exploit its power for catastrophic ends. The series balances action, humor, and emotional depth, offering a cohesive narrative that delves into the complexities of its characters and their relationships.