Minase Mai visits The World, a MMORPG, for the first time together with a friend, Tomonari Kasumi, using the hardware at the school club. Shortly thereafter she and her friend are found unconscious and are hospitalized. Mai regains consciousness quickly enough, but her friend stays in a coma. She decides to find out what caused this accident, for her sake and that of her friend. Mai suspects that the game had something to do with it, unthinkable, especially since 20 million users are playing it. But an encounter with a middle-aged man, Tokuoka Jun'ichirou, who had previously worked for the game company that created The World, confirms her worst suspicions. They decide to team up and investigate further...

— written by foo2

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Prequel: .hack//Sign
Sequel: .hack//Tasogare no Udewa Densetsu
Side story: .hack//Gift