In the year 2005, a computer virus called Pluto Kiss completely shuts down and restricts the global internet network of the world. In 2007, after extensive rescinding of the network, they create and release an online Network Game (MMORPG) called The World.

The anime focuses mainly around Tsukasa who obtains the power that damages the player in the real world. He suddenly awakens to find himself in The World, being able to feel, smell, touch, see and hear everything as if he was really there. He attempts to logout but fails to do so. Being unable to remember what has happened before he awoke, he wanders around aimlessly to find answers. While doing so, system moderators, the Scarlet Knights, suspect that he has been working with hackers, because of the tweaked character he was with. Tsukasa finds a magical item, that grants him the power to use this fearful Guardian that will protect him from harm.

.Hack//Sign is the first part of a large series of OVAs, TV Series, and Mangas based around The World. Included are .hack//GIFT(OAV),.hack//Intermezzo(OAV), .hack//Unison(OAV), .hack//Legend Of The Twilight(TV), and .hack//Liminality(OAV).