"Zero's Familiar" is a fantasy manga series set in a world where magic is a fundamental part of society, particularly within the prestigious Tristain Academy of Magic. The story centers on Louise Françoise Le Blanc de La Vallière, a young noblewoman nicknamed "Louise the Zero" due to her consistent failures in casting spells. During a crucial ritual where second-year students summon their familiar—a lifelong magical companion—Louise's spell goes awry, summoning Saito Hiraga, an ordinary Japanese high school student from Earth. Despite her disappointment, Louise is bound by tradition to accept Saito as her familiar, marking him with a runic seal that grants him enhanced abilities but also ties him to her will.

The dynamic between Louise and Saito forms the core of the narrative. Louise, haughty and prideful, struggles with her insecurities about her magical incompetence, while Saito, initially bewildered by his new reality, gradually adapts to his role as her familiar. Their relationship evolves from one of mutual frustration to a deeper, albeit tumultuous, bond as they face various challenges together. The story explores themes of identity, loyalty, and the clash between different worlds, as Saito's modern perspective often contrasts with the feudal and magical society of Halkeginia.

The plot expands beyond the academy as Louise and Saito become entangled in political intrigue, wars, and conflicts involving powerful factions. The Tristain Kingdom, where the academy is located, faces threats from rival nations like Albion and Gallia, as well as internal power struggles. Saito's unique abilities, granted by the runic seal, make him a valuable asset in these conflicts, but they also draw unwanted attention from those who seek to exploit his powers. The series incorporates elements of romance, action, and comedy, balancing lighthearted moments with high-stakes drama.

Key supporting characters include Kirche Augusta Frederica von Anhalt-Zerbst, a fiery and flirtatious classmate who often clashes with Louise; Tabitha, a quiet and skilled mage with a mysterious past; and Princess Henrietta of Tristain, whose friendship with Louise plays a significant role in the political subplots. The series also delves into the lore of Halkeginia, exploring the origins of magic, the role of familiars, and the connections between different realms, including Earth.

As the story progresses, Louise and Saito's relationship deepens, and they confront increasingly dangerous adversaries, including powerful mages, ancient artifacts, and even dragons. The narrative builds toward a climactic resolution that ties together the personal growth of the characters with the larger conflicts shaping their world. "Zero's Familiar" blends fantasy, romance, and adventure, creating a story that balances character-driven drama with expansive world-building.