"I'm Quitting Heroing" centers on Leo Demonheart, a former hero who saved the world from destruction but finds himself ostracized by humanity due to his overwhelming power. After defeating the Demon King and bringing peace, Leo is viewed as a threat rather than a savior, leading him to abandon his heroic role. Seeking a new purpose, he approaches the Demon Queen Echidna, offering his services to the demon army. Despite initial skepticism, Echidna agrees to let him join, intrigued by his potential and the opportunity to harness his abilities for the demons' benefit.

The series explores Leo's integration into the demon army, where he faces resistance from its members, who distrust humans and question his motives. Leo's unparalleled combat skills and strategic mind quickly prove invaluable, but his unorthodox methods and carefree demeanor create friction with the disciplined demon soldiers. Key characters include Echidna, the pragmatic and calculating Demon Queen; Mia, a young demon soldier who initially opposes Leo but gradually warms to him; and Alv, a high-ranking demon officer who struggles to reconcile his loyalty to Echidna with his distrust of Leo.

As the story progresses, Leo's presence begins to shift the dynamics within the demon army, challenging their traditional values and methods. His interactions with the demons reveal deeper layers of their society, including their struggles with identity, purpose, and the lingering scars of war. The narrative balances action and humor with moments of introspection, highlighting themes of redemption, acceptance, and the search for belonging. Leo's journey is not just about finding a new role but also about confronting the consequences of his past actions and redefining what it means to be a hero. The series maintains a steady pace, blending episodic challenges with an overarching plot that gradually unveils the complexities of its world and characters.