"Kusatta Kyōshi no Hōteishiki" centers on a high school mathematics teacher, Shougo Makabe, whose life takes a dark turn after a traumatic incident. Once a passionate educator, Makabe becomes disillusioned and adopts a nihilistic worldview, using his position to manipulate and exploit his students. His methods are unorthodox and often cruel, as he challenges his students to solve complex mathematical problems under extreme psychological pressure, believing that life itself is a series of unsolvable equations.

The narrative explores the lives of several students who become entangled in Makabe's twisted games. Among them is Akira Tachibana, a bright but emotionally fragile student who becomes a primary target of Makabe's psychological torment. Tachibana's struggle to maintain her sanity while navigating Makabe's challenges forms a central thread of the story. Other students, including the rebellious Kenta and the academically driven Yumi, also face Makabe's manipulations, each reacting differently to his methods. Their interactions reveal the broader impact of his actions on the school environment and their personal lives.

As the story progresses, Makabe's past is gradually unveiled, shedding light on the events that led to his descent into moral corruption. Flashbacks reveal his relationship with a former student, whose tragic fate serves as a catalyst for his current behavior. The series delves into themes of power dynamics, the consequences of trauma, and the ethical responsibilities of educators. The tension escalates as students begin to resist Makabe's influence, leading to confrontations that test their resilience and moral boundaries.

The plot builds toward a climactic resolution where the students must confront Makabe directly, challenging his worldview and seeking to reclaim their autonomy. The series concludes with an exploration of whether redemption is possible for those who have lost their way, leaving the audience to reflect on the complexities of human behavior and the potential for change.