"Zero's Familiar: Chevalier" is a fantasy manga series set in a world where magic and mythical creatures coexist. The story begins at the Tristain Academy of Magic, where Louise Françoise Le Blanc de La Vallière, a young noblewoman, struggles with her inability to cast spells correctly. Known as "Louise the Zero" due to her repeated failures, she is ridiculed by her peers. During a crucial ritual where students summon their familiars, Louise inadvertently summons Saito Hiraga, an ordinary Japanese high school student from Earth, instead of a magical creature. This event binds Saito to Louise as her familiar, marked by the runes on his hand.

The relationship between Louise and Saito is initially contentious, as Louise views Saito as inferior due to his commoner status, while Saito resents being treated as a servant. However, as the story progresses, their dynamic evolves through shared experiences and challenges. Saito discovers that his runes grant him unique abilities, such as enhanced strength and the capacity to wield powerful weapons, making him a valuable asset in their adventures.

The narrative expands beyond the academy as Louise and Saito become entangled in political intrigue and conflicts involving rival nations, ancient artifacts, and powerful magical entities. Key figures include Princess Henrietta of Tristain, who seeks Louise's assistance in matters of state, and Kirche Augusta Frederica von Anhalt Zerbst, a rival mage with a fiery personality. The series explores themes of loyalty, identity, and the clash between tradition and modernity, as Saito's presence disrupts the established order of the magical world.

Throughout the series, Louise and Saito face numerous adversaries, including the Reconquista rebellion, which threatens the stability of Tristain. Their journey is marked by battles, alliances, and personal growth, as they navigate their roles in a world where magic and technology intersect. The manga balances action, humor, and romance, with the evolving bond between Louise and Saito serving as a central narrative thread. The story concludes with resolutions to major conflicts, though the characters' relationships and the broader implications of Saito's presence in the magical world leave room for further exploration.