"The Isekai Returnee is Too OP for the Modern World" centers on a protagonist who, after being transported to a fantasy world and gaining immense power, returns to his original world. The story begins with the protagonist, a seemingly ordinary individual, being summoned to a magical realm where he undergoes rigorous training and battles, eventually becoming one of the most powerful beings in that world. Upon his return to Earth, he discovers that his abilities have not diminished, making him overwhelmingly powerful compared to the ordinary humans around him.

The narrative explores the challenges and consequences of his newfound strength in a modern setting. While he attempts to reintegrate into everyday life, his extraordinary abilities attract attention from various factions, including government agencies, secret organizations, and otherworldly entities. These groups seek to either exploit his powers or eliminate him as a potential threat. The protagonist must navigate these conflicts while trying to maintain a semblance of normalcy, often relying on his strategic thinking and combat experience from the other world.

Key characters include the protagonist, whose calm demeanor and tactical prowess contrast with his overwhelming power; allies from his time in the fantasy world who occasionally cross over to assist him; and adversaries who view him as a destabilizing force. The story balances action-packed sequences with moments of introspection, as the protagonist grapples with the moral implications of his abilities and the impact they have on those around him. Themes of power, responsibility, and the clash between fantasy and reality are central to the narrative, creating a dynamic and engaging plot.