G.U.+ follows the player Haseo, a powerful PKKer known as "The Terror of Death". Haseo is obsessed with an infamous PKer known only as "Tri-Edge" who attacked a close friend of his sometime in the past. The manga details Haseo's search for Tri-Edge, and his eventual inclusion in a mysterious group known as "G.U."

Back story of the ps2 game (same title)

In 2015 a fire at the main CC Corp. building resulted in the loss of a lot of data, and The World was forced to shut down. By splicing in data from what would have been another game, CC Corp was able to rebuild the game system. The next year, "The World R:2" was released, but many old players objected because they were unable to bring over their old characters. However, many new users were drawn in by the new style, and over 12 million copies were sold. Though not a match for the previous version's 20 million users, it became the top-selling netgame on the market." As a result of the new world, four major clans (Kestrel, Lunar Tree, Canard, and Raven) came to power. The clans actions shape 'The World', and most players end up taking one side or another during conflicts.