What if real life could "glitch?"

Reika and Youko, two students, wander in a huge building, a real never-ending labyrinth defying imagination. Their only clue: a mysterious message on a table signed by Tagami, a famous video game creator who seems to know more about this place.

What is the real nature of this place? Why are Reika and Youko humanity's only hope?

(Source: MU)