"Sore ga Kanojo no Seigi nara" centers on the life of Katsuragi, a high school student who becomes entangled in a series of morally complex situations after encountering a mysterious girl named Rin. Rin possesses a unique ability: she can transform into a powerful, justice-driven entity known as "Seigi," which translates to "Justice." This transformation is triggered when she witnesses acts of injustice or cruelty, compelling her to intervene and punish the wrongdoers. However, her methods are often extreme, blurring the line between justice and vengeance.

Katsuragi, initially a bystander, is drawn into Rin's world after witnessing her abilities firsthand. He becomes her confidant and ally, grappling with the ethical dilemmas posed by her actions. As the story progresses, the duo faces various antagonists, each representing different facets of societal corruption, from bullying and exploitation to systemic abuse of power. The narrative explores themes of morality, the nature of justice, and the consequences of taking the law into one's own hands.

Rin's backstory is gradually revealed, shedding light on the origins of her powers and her personal motivations. Her past is marked by trauma and loss, which fuels her relentless pursuit of justice. Katsuragi, on the other hand, serves as a grounding force, often questioning the righteousness of Rin's methods and striving to find a balance between her ideals and the realities of human imperfection.

The series delves into the psychological and emotional toll of Rin's mission, highlighting the isolation and burden she carries as someone who feels compelled to act as a self-appointed arbiter of justice. Katsuragi's evolving relationship with Rin adds depth to the narrative, as he struggles to support her while grappling with his own moral compass.

The plot is driven by episodic confrontations with various wrongdoers, interspersed with moments of character development and introspection. The series maintains a tense, action-packed atmosphere while also addressing philosophical questions about the nature of justice and the limits of personal responsibility. The dynamic between Rin and Katsuragi forms the emotional core of the story, as they navigate the complexities of their partnership and the challenges posed by their shared mission.