"I'm Quitting Heroing" centers on Leo, a former hero who once saved the world from the Demon King but finds himself ostracized by humanity due to his overwhelming power. After being labeled a threat, Leo decides to abandon his heroic role and seeks refuge in the Demon King's army, now led by the Demon King's daughter, Echidna. The story explores Leo's integration into the demon ranks, where he uses his unparalleled combat skills and strategic mind to assist the demons in rebuilding their society and defending against external threats.

The narrative delves into themes of redemption, identity, and the complexities of morality. Leo's journey is marked by his efforts to reconcile his past as a hero with his new life among former enemies. Echidna, initially wary of Leo, gradually comes to trust him as he proves his loyalty and contributes to the demons' survival. The series also introduces a diverse cast of characters, including other former heroes and demons, each with their own motivations and struggles.

The plot balances action, humor, and introspection, as Leo navigates the challenges of his new role while confronting remnants of his past. The demons' society, once seen as purely antagonistic, is portrayed with nuance, highlighting their struggles and aspirations. As the story progresses, alliances shift, and new threats emerge, forcing Leo and his companions to confront their beliefs and adapt to an ever-changing world. The series maintains a steady pace, blending episodic adventures with an overarching narrative that explores the consequences of war, the nature of heroism, and the possibility of coexistence between former adversaries.