"Saga of Tanya the Evil" is set in an alternate world resembling early 20th-century Europe, where magic is a tangible force harnessed by individuals known as mages. The story centers on Tanya Degurechaff, a ruthless and calculating soldier who serves as an officer in the Imperial Army. Tanya is, in fact, the reincarnation of a Japanese salaryman who, after being killed by a disgruntled subordinate, encounters a mysterious entity referred to as "Being X." This entity, claiming to be God, challenges the salaryman's atheism and reincarnates him into a war-torn world as a young girl, forcing him to confront faith and survival in a hostile environment.

Tanya's reincarnation retains her memories and intellect, allowing her to excel in military strategy and combat. Despite her childlike appearance, she is a cold and pragmatic tactician, driven by a desire to defy Being X and secure her own survival. The narrative unfolds during a global conflict reminiscent of World War I, with the Empire—a powerful nation resembling Imperial Germany—fighting against multiple enemies on various fronts. Tanya rises through the ranks of the Imperial Army, earning a reputation as a prodigious mage and a fearsome commander. Her unit, the 203rd Aerial Mage Battalion, becomes instrumental in the Empire's military campaigns.

The series explores the moral ambiguities of war, the psychological toll of combat, and the clash between Tanya's rational, self-serving worldview and the spiritual challenges posed by Being X. Tanya's interactions with her subordinates, superiors, and enemies highlight her manipulative nature and unwavering focus on efficiency and victory. Key characters include Viktoriya Ivanovna Serebryakov, Tanya's loyal adjutant, who provides a humanizing contrast to Tanya's ruthlessness, and Major Erich von Rerugen, a high-ranking officer who harbors deep suspicions about Tanya's true nature.

As the war escalates, Tanya's actions often lead to unintended consequences, further entangling her in the machinations of Being X. The series balances intense aerial battles, strategic warfare, and philosophical undertones, creating a narrative that is both action-packed and thought-provoking. The interplay between Tanya's relentless ambition and the existential challenges she faces forms the core of the story, making "Saga of Tanya the Evil" a compelling exploration of power, faith, and survival in a world ravaged by conflict.