"Mahōka Kōkō no Rettōsei: Double Seven-hen" is a manga adaptation of the light novel series "The Irregular at Magic High School," focusing on the events of the "Double Seven Arc." The story is set in a world where magic is systematized through modern technology, and the narrative centers on Tatsuya Shiba and his sister Miyuki, students at First High School, a prestigious institution for magicians.

The arc begins with the arrival of the new school year, marked by the Double Seven Festival, a significant event where students showcase their magical abilities. Tatsuya, despite being labeled as a "weed" due to his low practical magic scores, possesses exceptional technical knowledge and combat skills. Miyuki, on the other hand, is a top-tier "bloom," admired for her powerful magical abilities and elegance. Their relationship, while outwardly sibling-like, carries underlying complexities due to their family's secretive background and the emotional bonds they share.

The plot thickens when a series of incidents disrupt the festival. A mysterious group targets the school, aiming to sabotage the event and undermine the reputation of magicians. Tatsuya, often underestimated by his peers, takes on a central role in uncovering the conspiracy. His unique abilities, including his "Decomposition" and "Regrowth" powers, become crucial in countering the threats. Meanwhile, Miyuki's leadership and magical prowess help rally the students to defend their school.

Key characters include Erika Chiba, Leo Saijou, and Mizuki Shibata, who support Tatsuya and Miyuki in their efforts. Erika, a skilled swordswoman, and Leo, a physically strong combatant, provide tactical assistance, while Mizuki, though less combat-oriented, contributes through her perceptiveness and loyalty. The antagonists, including members of the anti-magic faction, are driven by ideological motives, seeking to expose the flaws in the magician-dominated society.

The narrative explores themes of power, discrimination, and loyalty, as Tatsuya navigates the challenges posed by both external threats and internal school dynamics. The arc culminates in a high-stakes confrontation, where Tatsuya's strategic mind and Miyuki's magical strength are put to the test. The resolution reinforces their bond and highlights the complexities of their world, setting the stage for future developments in the series.