"The Great Jahy Will Not Be Defeated!" centers on Jahy, the former second-in-command of the Demon Realm, who finds herself stranded in the human world after a magical girl destroys the Demon Realm's mana crystal. Stripped of her immense power and reduced to a childlike form, Jahy struggles to adapt to her new life while plotting to restore the Demon Realm and regain her former glory. Despite her grandiose ambitions, Jahy's efforts are consistently thwarted by her lack of resources, her unfamiliarity with human society, and her own comedic ineptitude.

Jahy's daily life revolves around mundane tasks such as finding affordable housing, working part-time jobs, and managing her limited finances, all while attempting to gather fragments of the shattered mana crystal. Her interactions with humans and other supernatural beings often lead to humorous and chaotic situations. Key characters include her landlord, a kind but oblivious woman who provides Jahy with a place to stay, and Saurva, a former subordinate who now works at a café and occasionally assists Jahy in her endeavors. The magical girl responsible for the destruction of the mana crystal also reappears, adding tension and conflict to Jahy's mission.

The series balances comedic elements with moments of character growth, as Jahy gradually learns to navigate the human world and forms unexpected bonds with those around her. Her pride and determination to reclaim her status as a powerful demon are juxtaposed with her vulnerability and the challenges of her new reality. The narrative explores themes of resilience, adaptation, and the contrast between ambition and humility, all while maintaining a lighthearted and episodic structure.