"Overlord" is set in a virtual reality MMORPG called Yggdrasil, where players immerse themselves in a fantasy world filled with magic, monsters, and guilds. The story begins as the game's servers are about to shut down permanently. Momonga, the leader of the powerful guild Ainz Ooal Gown, decides to stay logged in until the very end. However, when the shutdown time passes, he finds himself still in the game, now transformed into his in-game avatar: an immensely powerful skeletal sorcerer. The world around him has also changed, no longer resembling the game he knew but instead feeling like a fully realized reality.

Momonga, now adopting the name Ainz Ooal Gown, discovers that the NPCs (non-player characters) of his guild's headquarters, the Great Tomb of Nazarick, have gained sentience and personalities. These NPCs, including the loyal and intelligent Albedo, the battle-crazed Cocytus, and the stoic Demiurge, revere Ainz as their supreme ruler. Determined to uncover the truth behind his predicament and the changes to the world, Ainz sets out to explore this new reality while maintaining the facade of an all-powerful overlord to ensure the loyalty of his subordinates.

As Ainz ventures beyond Nazarick, he encounters various factions, kingdoms, and individuals, each with their own agendas and power structures. His actions, often driven by a mix of curiosity, caution, and a desire to protect Nazarick, have far-reaching consequences. Ainz's immense power and the loyalty of his NPCs allow him to influence the world significantly, whether through diplomacy, subterfuge, or outright conquest. However, his human emotions and moral dilemmas occasionally surface, creating tension between his role as a ruler and his lingering humanity.

The narrative explores themes of power, identity, and the consequences of absolute authority. Ainz's journey is marked by strategic planning, alliances, and conflicts, as he navigates a world where his actions can reshape entire civilizations. The series balances moments of intense action with intricate world-building, showcasing the complexities of the new reality Ainz inhabits and the challenges he faces as both a leader and an individual.

Key characters include Ainz Ooal Gown, the protagonist whose dual nature as a powerful undead and a former human drives much of the story; Albedo, the Overseer of the Guardians, whose unwavering devotion to Ainz often leads to morally ambiguous decisions; and Demiurge, a cunning strategist who aids Ainz in expanding Nazarick's influence. The interactions between these characters and the world around them form the core of the narrative, blending elements of fantasy, strategy, and psychological exploration.