PLOT

Sword Art Online (SAO) is a Virtual Reality Massively Multiplayer Online Role-Playing Game (VRMMORPG), released in 2022. With the Nerve Gear, a virtual reality helmet that stimulates the user's five senses via their brain, players can experience and control their in-game characters with their minds.
On November 6, 2022, the players log in for the first time, and later discover that they are unable to log out. They are then informed by Akihiko Kayaba, SAO's creator, that if they wish to be free, they must reach the 100th floor of the game's tower and defeat the final boss. However, if their avatars die in-game, their bodies will also die in the real world. One of these players is Kazuto "Kirito" Kirigaya, a skilled player who is determined to beat the game. As the game progresses for two years, Kirito eventually befriends a female player named Asuna with whom he ultimately falls in love. After the duo discover the identity of Kayaba's avatar Heathcliff in SAO, they confront and destroy him, freeing themselves and the other players from the game.

SAO was closed, but then, the game bought by a new company called ArtsNerve, The previous SAO was fixed, and upgraded. Now, it's called SAO : Rebirth or SAO: R for short. SAO:R was released in 2027 with the new version of NerveGear, called AmuSphere. People bought the game, and on the first day of the release, 4 million copies sold out, and 4 million new players arrived at the game. Among those players, 15 beta testers who tried the game in Close Beta. 

The game was new, so some bugs occurred. The AmuSphere wasn't compatible with SAO:R, the AmuSphere caused a big problem, 5 % of the players fell into comatose, and a big electromagnetic ball appeared near the AmuSphere, Suddenly the players find themselves transported into the virtual game world and donning their in-game avatars, and they can't log out. ArtsNerve tried to fix this, but it's too late 4 million people was transported into the game, and if ArtsNerve shut the server down, 4 million people will be killed. If you die in the game, you will die also in real life. 

WELCOME TO SWORD ART ONLINE: REBIRTH!