.Hack takes place in a fictional MMORPG (Massively Multiplayer Online RolePlaying Game) called "The World". In The World, players are free to communicate with each other, form parties to battle monsters, or just simply hang out and have fun. Much like life, The World has rules and priorities, figuratively speaking - the Crimson Knights. The Knights are players that help keep balance and peace in the world, and monitor any abnormalities while seeking to fix them, and report anything beyond their control to the programmers that run The World. Tsukasa, to the Knights, is considered to be an anomaly as he is soon discovered as someone incapable of escaping the virtual reality and rejoining the real world. Because of this, he is pursued during his first bit of time here until something much more formidable and dangerous is discovered behind Tsukasa's inability to log out. As Tsukasa begins making friends who want to help him, terrible things begin to occur in The World. Something must be done, or Tsukasa may be stuck in this reality forever, and The World may be forced to shut down.