"Overlord: The Undead King Oh!" is a manga adaptation of the light novel series "Overlord," written by Kugane Maruyama and illustrated by so-bin. The story is set in a virtual reality MMORPG called Yggdrasil, which is on the verge of shutting down. The protagonist, Momonga, is a powerful undead sorcerer and the guild master of Ainz Ooal Gown, one of the most renowned guilds in the game. As the servers are about to close, Momonga decides to stay logged in until the very end. However, when the shutdown time passes, he finds himself still in the game world, which has now become a fully realized reality.

Momonga, now trapped in the body of his skeletal avatar, discovers that the NPCs 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 Shalltear, and the stoic Cocytus, revere Momonga as their supreme ruler. Determined to uncover the truth behind his predicament and the changes in the world, Momonga adopts the name Ainz Ooal Gown and sets out to explore this new reality while maintaining the facade of an all-powerful overlord to protect his guild's legacy.

The narrative follows Ainz as he navigates this unfamiliar world, encountering various factions, kingdoms, and powerful entities. His actions are driven by a mix of curiosity, strategic planning, and a desire to ensure the safety and dominance of Nazarick. Along the way, Ainz forms alliances, engages in battles, and manipulates events to his advantage, often relying on his vast knowledge of Yggdrasil's mechanics and the loyalty of his NPC subordinates.

The story delves into themes of power, loyalty, and the consequences of absolute authority. Ainz's internal conflict between his human emotions and his undead nature adds depth to his character, while the interactions between the NPCs and the inhabitants of the new world provide a rich tapestry of relationships and conflicts. The manga captures the intricate world-building and complex character dynamics of the original light novel, presenting a tale of an undead king striving to understand and conquer a world that has become his new reality.