.Hack//G.U. Vol. 1: Rebirth

.hack//G.U. takes place in an alternate version of Earth in the year 2017. As depicted in the first .hack game series, the "2nd Network Crisis" was an incident that occurred seven years ago in which many computer systems across Japan malfunctioned. Through the efforts of those games' hero, Kite, the incident resulted in the birth of Aura, the ultimate artificial intelligence (AI), capable of making decisions for itself. Under the guidance of Aura, The World, the most popular massively multiplayer online role-playing game at the time, flourishes and the events of .hack//Legend of the Twilight occur during this four-year period.

In late 2014, Aura disappears, resulting in the slow decay of The World and the Internet as a whole. In response, the company that administrates The World, CC Corp, sets up "Project G.U." and tasks them with remedying the problem. They come up with the "Restore Aura (RA) Plan", in which they attempt to retrieve the fragments of Morganna—an AI that served as Aura's "mother"—and seal them into player characters (PCs) of The World. In theory, this would allow the Project G.U. programmers to manipulate the Morganna program to recreate Aura. Only certain "chosen" players could control these special "Epitaph PCs".

After locating candidates to operate the Epitaph PCs, the team proceeds to test the RA Plan despite concerns over "anti-existences" that might damage the Internet. The program fails catastrophically and destroys over 80% of the data for The World.[9] Instead of trying to salvage the data, CC Corp develops The World R:2 and releases it to a new generation of players. However, this new game is less popular than its predecessor and becomes plagued by "player killers" (PKs), players who target other players for fun and sport. The Epitaph PCs which had been lost during the RA Plan disaster resurface in the new version, attached to certain players of interest.

Vol 1: Rebirth:
The story starts with Haseo who, eight months after starting playing in The World, has become a powerful player killer killer or PKK and searches for Tri-Edge, responsible for killing his friend Shino's character and leaving the player behind her in a coma. Tri-Edge uses an illegal skill on Haseo, which resets his character data to level one. Lacking the strength he once had, Haseo is mistaken for a newbie player by Silabus and Gaspard, who invite him to join their guild, Canard, eventually becoming its Guild Master. During this, he is approached by Kuhn and Pi, two players associated with G.U., a group run by Yata who works for CC Corp, and is investigating AIDA, a bizarre computer anomaly that is appearing in the game. Yata requests Haseo's help as he is an Epitaph User, and is one of the eight players who can use an Avatar, the only vehicle to combat AIDA. Haseo agrees on the condition that Yata find Tri-Edge for him, which Yata agrees with.

During this time he is repeatedly contacted by Atoli, a gentle healer that tries to convince Haseo to enjoy the game and its world more, though Haseo is annoyed by her resemblance to Shino. Haseo also watches an arena match between Endrance, Arena Emperor, and a challenger, and is startled to see Endrance, a player he saw with AIDA, use an Avatar. Determined to learn more, he joins the arena and eventually defeats Endrance. After winning, Atoli, on advice from Ovan, finds a Tri-Edge sign, a red carving, and transports to a region outside the normal game parameters, determined to get Haseo to see her as valuable. Realizing she is in great danger, Haseo, Pi, and Kuhn locate her struggling to open a locker. After distracting her, Tri-Edge appears, and is defeated by Haseo. After doing so, an AIDA attacks Atoli's character.

.Hack//G.U. simulates a massively multiplayer online role-playing game; players assume the role of a participant in a fictional game called The World. While in The World, the player controls the on-screen player character, Haseo, from a third-person person perspective (with optional first-person mode). The player may control the camera using the game controller's right analog stick. Within the fictional game, players explore monster-infested fields and dungeons as well as "Root Towns" that are free of combat. They also can "log-off" from the game and return to a computer desktop interface which includes in-game e-mail, news, and message boards, as well as desktop and background music customization options. In Reminisce, an optional card game called "Crimson VS" becomes available. The player may save the game to a memory card both from the desktop and within The World at a Save Shop. After the player completes the game, a Data Flag appears on the save file, which allows the transfer of all aspects of the player character and party members to the next game in the series. This can also be applied to previous games if the player first finished the Reminsce or Redemption.

Players can attack monsters in real time. However, the action pauses whenever the menu is opened in order to select magic to cast, items to use, or skills to perform. The player only directly controls Haseo, while the other characters are controlled by artificial intelligence. The player may either provide guidelines ("Free Will", "Rage", "Life", etc.) or issue direct commands (for example, to cast a particular magic at a particular enemy) to the computer-controlled characters. Monsters roam the environments freely or guard treasure chests. Once combat is initiated, via a "surprise attack" or the player being spotted, a wall erects around the combat area to prevent escape. Weapons give the player access to the Skill Trigger feature which allows them to perform a powerful attack at the cost of losing SPs. As an adept rogue character, Haseo can wield multiple weapons for close combat. Although Haseo can only use one weapon per battle, during Reminisce the player can change weapons in the middle of Skill Triggers. Under certain conditions, the player may execute an enhanced type of Skill Trigger called "Rengeki". These attacks deal greater damage and fill a Morale gauge, which allows the player and his team members to use a combination attack. Certain plot-related fights, called "Avatar battles", use a different interface which incorporates shoot 'em up gameplay elements. After depleting the opponent's health, players must charge up a "Data Drain" to end the battle.

Root Towns are non-combat areas in The World. The player may restock on items, buy equipment, or chat and trade with other "players" of The World. The player may also undertake optional quests and visit guilds. A key feature of all towns is the Chaos Gate. This blue portal is used to travel between towns (called "servers") as well as access the fields and dungeons where battles take place. A password system controls the characteristics of each area. Depending on the characteristics of each word in the three word phrase, the resulting area may have different attributes such as prevalence of monsters or items, among other features.