Mai and her boyfriend, Tomonari Kasumi (known to The World as Sieg), are taken to a hospital due to both of them collapsing while playing a MMORPG called The World. Despite Mai waking up shortly after arriving at the hospital, Tomonari remains in a coma. For this, Mai is the object of rumors, especially when the clubhouse (known as G-study) is closed after the incident. It is soon revealed that she heard a certain sound (which is also present at important points in .hack//Sign) shortly before collapsing, and she later discovers that it is A in C major, which is used to tune instruments, including her violin. It was this sound that enabled her to wake up, and it was the first sound she made when she did. She meets Junichiro Tokuoka, and although she distrusts him at the beginning, as they investigate into Tomonari's coma, they soon become allies, if not friends. The investigation itself starts with Tokuoka accessing Tomonari's (Sieg's) account on The World, with Mai providing the password. Tokuoka accidentally opens the e-mail, and discovers that two users, Yukichin and Kyo, are close with Tomonari and have been harassing him with e-mails regarding his absence. It is also revealed during the course of the episode that Tomonari took the name Sieg as a contraction of Siegfried, from the German operatic cycle Der Ring Des Nibelungen, which was written by Wagner. Balmung is the name of Siegfried's sword, and Tomonari told Mai that he would become the possessor rather than the sword: a reference to the fact that he aimed to surpass the player character Balmung of the Azure Sky, one of the Descendants of Fianna. Tokuoka and Mai's investigation leads them to break into G-study, in order to attempt to recreate the same situation so that they can find out why Mai and Tomonari collapsed. The sound starts up again when they enter a field, and although Mai saves herself, Tokuoka seems to be losing it. In desperation, Mai destroys the monitor and (presumably) computer that they w

The other players are in a new realm, a place where old and lost data is kept. Subaru's waiting pays off and Tsukasa finally appears and they go through the gate. Our players learn a lot from Helba, and start to formulate a plan on what to do next. Subaru and Tsukasa bond more, and learn more about each other. They figure out that the enemy is after Subaru and they need to protect her, a battle ensues. Tsukasa makes the decision that the does want to log out.

Junichiro Tokuoka travels to the city of Hida-Takayama to meet up with a miss Kyoko Tohno to discuss further details surrounding the mysteries in The World as well as the legends surrounding The Epitaph of Twilight. After receiving a phone call from a person working under the alias Bith the Black, Tokuoka and Kyoko realize that they are being watched by Helba's agents. A series of messages from Bith lead them to numerous historical sites throughout the city. As several of the messages come from e-mails sent to Mai Minase and Yuki Aihara, Tokuoka suspects that Helba has been following their actions for quite some time. The team begins to eventually put together the puzzle behind the mysterious woman of Emma Wielant the author of the Epitaph. After a chance run in with Kyoko's Father, they discover that he was contacted by Bith as well, working under another alias "Ichiro Sato".Junichiro Tokuoka travels to the city of Hida-Takayama to meet up with a miss Kyoko Tohno to discuss further details surrounding the mysteries in The World as well as the legends surrounding The Epitaph of Twilight. After receiving a phone call from a person working under the alias Bith the Black, Tokuoka and Kyoko realize that they are being watched by Helba's agents. A series of messages from Bith lead them to numerous historical sites throughout the city. As several of the messages come from e-mails sent to Mai Minase and Yuki Aihara, Tokuoka suspects that Helba has been following their actions for quite some time. The team begins to eventually put together the puzzle behind the mysterious woman of Emma Wielant the author of the Epitaph. After a chance run in with Kyoko's Father, they discover that he was contacted by Bith as well, working under another alias "Ichiro Sato". Over lunch Ichiro explains that Harald Hoerwick had used the game Fragment, the precursor to The World, to create a tribute to Emma's Epitaph of Twilight. But that alone wouldn't explain the current situation facing The W

Several weeks after the meeting in Hida-Takayama on Christmas Eve, Junichiro Tokuoka, Kyoko Tohno, Yuki Aihara, and Mai Minase all gather on a dinner boat in Tokyo Bay, to prepare for their risky (and most certainly illegal) mission. Despite their nervousness, each one is dedicated to helping Ichiro Sato complete the mission sent from Helba. Ichiro himself soon arrives on a different boat. While traveling to their destination, Ichiro reveals the true nature of the problems that have been plaguing The World. Harald Hoerwick's program, Fragment, had been nothing more than a program to create an "Ultimate AI". The system to create this A.I. had somehow become self autonomous, and was working to delay the birth of the A.I. known as Aura, to keep itself from finishing its purpose and ceasing to exist. If the problems are to be solved, the Ultimate A.I. must be born, and it's up to Tokuoka and his team to help that event occur.