"Mahōka Kōkō no Rettōsei: Tsuioku-hen" is a prequel manga to the main series "Mahōka Kōkō no Rettōsei," focusing on the backstory of Tatsuya and Miyuki Shiba, the central siblings. Set in a world where magic is systematized through modern technology, the story delves into the siblings' childhood and their experiences within the Yotsuba Clan, one of the Ten Master Clauses, a group of powerful magical families in Japan.

The narrative begins with Tatsuya and Miyuki living under the strict and often oppressive control of the Yotsuba Clan. Tatsuya, despite being born with immense magical potential, is deemed a failure due to his inability to perform conventional magic, leading to his designation as a "Guardian," a role that prioritizes combat and protection over traditional magical abilities. Miyuki, on the other hand, is celebrated as a prodigy, her talents making her a key figure within the clan. The siblings share a close bond, with Miyuki deeply admiring Tatsuya for his strength and resilience despite his perceived shortcomings.

The plot explores the harsh training and psychological conditioning Tatsuya undergoes to fulfill his role as a Guardian. His abilities, particularly his unique skills in decomposition and regeneration, are honed through rigorous and often brutal methods. Meanwhile, Miyuki's training focuses on refining her natural talents, preparing her for a future leadership role within the clan. The story highlights the siblings' struggles with the expectations and manipulations of the Yotsuba Clan, as well as their unwavering loyalty to each other.

Key events include Tatsuya's involvement in a mission to rescue Maya Yotsuba, the clan's leader, from a hostile faction. This mission serves as a turning point, showcasing Tatsuya's strategic brilliance and combat prowess, which earn him a grudging respect from the clan. The narrative also touches on the political intrigue within the Yotsuba Clan, revealing the complex dynamics and power struggles that shape the siblings' lives.

The manga provides insight into the origins of Tatsuya's stoic demeanor and Miyuki's devotion to her brother, setting the stage for their roles in the main series. It also introduces other significant characters, such as Maya Yotsuba, whose actions and decisions have a profound impact on the siblings' lives. The story is marked by themes of loyalty, sacrifice, and the consequences of power, offering a deeper understanding of the characters and their motivations.