Set in a fictional MMORPG called The World, the series follows twins Shugo and Rena. After winning a contest, Rena is given a pair of chibi avatars in the design of the legendary .hackers, Kite and BlackRose. After an odd occurrence, a mysterious AI named Aura gives Shugo the Twilight Bracelet, an item that both aids and hinders him. Rena and Shugo embark on an adventure to find Aura and unravel the mystery of the Twilight Bracelet.
Along the way, Shugo and Rena befriend Mireille, a rare item hunter; the fierce Ouka,;[notes 1] and Hotaru, a peaceful girl. While waiting for an event to start, they encounter a strange girl named Zefie, who is lured to Shugo because she believes the bracelet smells like her mother, Aura. It is later realized that Zefie is a vagrant AI – an AI that acts independently outside the parameters of the game. Zefie's presence upsets many, including the Cobalt Knights, a group of administrators that follow the rule "If you can't control it, delete it" to an extreme.