"The Irregular at Magic High School: Reminiscence Arc" serves as a prequel to the main series, delving into the backstory of the Shiba siblings, Tatsuya and Miyuki, and their relationship prior to the events of the original story. Set in a world where magic is systematized through advanced technology, the arc explores the siblings' lives during their time at First High School, focusing on their bond and the challenges they face.

The narrative begins with Tatsuya, a technically skilled but magically underpowered student, and Miyuki, a prodigious magician, navigating the hierarchical and competitive environment of the magic high school. Despite Tatsuya's perceived lack of magical prowess, his strategic mind and combat abilities make him a formidable figure. Miyuki, on the other hand, is revered for her exceptional magical talent and grace, often drawing attention and admiration from her peers. The siblings share a deep, unbreakable bond, which is central to the story.

The arc highlights key events that shape their relationship and their roles within the school. Tatsuya's involvement in a covert military operation, known as the "Dahan Incident," is a pivotal moment. This mission reveals his true capabilities and the extent of his loyalty to Miyuki, as he risks his life to protect her. The incident also underscores the political tensions and power struggles within the magical community, as various factions vie for control and influence.

Throughout the arc, the siblings face external threats and internal conflicts, including the scrutiny of their peers and the expectations placed upon them by their family and society. Miyuki's growing awareness of her feelings for Tatsuya adds a layer of complexity to their relationship, as she grapples with the societal norms and taboos surrounding their bond. Tatsuya, ever protective and pragmatic, remains focused on ensuring Miyuki's safety and happiness, even as he confronts his own limitations and the harsh realities of their world.

The "Reminiscence Arc" provides crucial context for the main series, offering insights into the origins of the siblings' dynamic and the events that shape their personalities and motivations. It also introduces key supporting characters, such as their classmates and allies, who play significant roles in the broader narrative. By exploring the siblings' past, the arc enriches the understanding of their actions and relationships in the main storyline, while maintaining the series' blend of action, intrigue, and emotional depth.