From the introduction:
In the year 2138, virtual reality gaming is booming. YGGDRASIL, a popular online game strongly influenced by nordic and other well known myths, is a DMMO-RPG (Dive Massively Multiplayer Online Role Playing Game) developed in Japan, serviced from 2126 to 2138 AD. One day it is quietly shut down. However, one player named Momonga decides not to log out. As the game shuts down, Momonga is then transformed into his avatar, a skeleton wizard, and becomes "the most powerful magic caster in the world.” However it is not the YGGDRASIL, nor the Nine Worlds based within it, that he knew but instead a New World never before seen. As the world continues to change, with the non-player characters (NPCs) of The Great Tomb of Nazarick beginning to feel emotion and become living beings, Momonga decides on his course of action: Having no parents, friends, or place in society, Momonga decides to conquer the new world the game has become.