.hack//G.U. Vol. 1: Rebirth takes place three years after .hack//QUARANTINE ended. One year after "The Twilight Incident", a huge fire started in CC Corp destroying most of the original "The World" servers and forcing them to shut the game down soon after. A new version of the "The World" was created one year after the fire called "The World R:2". You play as Haseo, a PKK (Player Killer Killer) searching for the player killer who Data Drained his friend into a coma, Tri-Edge.

The player meets reliable friends in Gaspard, Silabus, Atoli, Kuhn and many others later in game. The battle system is still real-time but with mostly hack and slash action. Also, when the player lands a certain number of combos on an enemy and they glow blue and yellow you can perform special Rengeki moves. Rengeki moves get more powerful as you progress through the game. The player also has a morale gauge in the top right corner of your screen. When the gauge is full, the player can perform an Awakening attack with help from his/her party members. There are two types of Awakening attacks, Demon and Beast Awakening. The Demon Awakening lets you unleash powerful magic attacks without using SP (Skill Points). The Beast Awaking makes the player invulnerable to physical attacks for a set period of time while enhancing his/her physical abilities.

New features added are guilds, arena battles and weapon customization. Guilds are groups people join for various reasons. The player can join at least two this game. Arena battles are basically two teams of three characters battling each other for fun or for championships. Weapon customization can be done with items the player gets on travels or buys in shops to element or status effect to your weapons.