"The Hero Life of a (Self-Proclaimed) 'Mediocre' Demon!" centers on Demon Lord Astaroth, a powerful and feared ruler of the demon world. Despite his immense strength and reputation, Astaroth harbors a deep dissatisfaction with his life, feeling unfulfilled and bored by the monotony of his role. Seeking a change, he decides to abandon his position and live as an ordinary human, adopting the name "Asta" and moving to the human world.

In the human realm, Asta attempts to lead a quiet, unremarkable life, working mundane jobs and avoiding attention. However, his extraordinary abilities and demonic nature frequently complicate his efforts. His immense strength, magical prowess, and unique appearance often draw unwanted attention, making it difficult for him to blend in. Despite his attempts to remain inconspicuous, Asta finds himself inadvertently involved in various conflicts and adventures, often helping others while trying to maintain his low profile.

The narrative explores Asta's interactions with humans and other supernatural beings, highlighting the contrast between his desire for a simple life and the chaos that follows him. Key characters include Asta's former subordinates from the demon world, who occasionally seek him out for guidance or assistance, and humans who become entangled in his life. These relationships add depth to the story, as Asta navigates his dual identity and the challenges of living between two worlds.

Themes of identity, purpose, and the search for meaning are woven throughout the series, as Asta grapples with his past as a Demon Lord and his present as a self-proclaimed "mediocre" human. The manga balances humor, action, and introspection, offering a blend of lighthearted moments and more serious reflections on Asta's journey. The story progresses through episodic arcs, each contributing to Asta's growth and the gradual unfolding of his character, while maintaining a focus on his quest for a fulfilling life beyond his former glory.