In 2022, a Virtual Reality Massively Multiplayer Online Role-Playing Game (VRMMORPG) called Sword Art Online (SAO) is released. With the NerveGear, a 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, 10,000 players log into the SAO’s mainframe cyberspace for the first time and discover that they are unable to log-out. They quickly discover from Akihiko Kayaba, the creator of SAO and NerveGear, they must reach the 100th floor of the game’s castle and defeat the final boss if they wish to be free. Those who suffer in-game destruction or remove the NerveGear out-of-game will result in their real-life deaths.
One of the players, Kazuto “Kirito” Kirigaya, is one of the 1,000 beta testers in the game’s previous closed beta. With the advantage of previous VR gaming experience and a drive to protect other beta testers from discrimination he isolates himself from the greater group and plays the game alone, bearing the mantle of “beater”, or “beta cheater”. As the players progress through the game Kirito eventually befriends a young girl named Asuna Yūki, forming a relationship with and later marrying her in-game. After the duo discover the identity of Kayaba’s program Heathcliff in SAO, who was discovered to be the leader of the guild Asuna joined in, they confront and destroy him, freeing themselves and the other players from the game.
In the real world, Kirito discovers that 300 SAO players, including Asuna, have remained trapped in their NerveGear. Several months later, he is informed by Agil, another SAO survivor, that a figure similar to Asuna was spotted on a giant tree (known as “The World Tree“) in another VRMMORPG cyberspace called Alfheim Online (ALO). Assisted in-game by his cousin Suguha “Leafa” Kirigaya, he quickly learns that the trapped players in ALO are part of an evil plan conceived by Nobuyuki Sugō to perform illegal experiments on their minds. The ultimate goal is to perfect mind-control for financial gain and to subjugate Asuna, whom he intends to marry in the real world, to assume control of her family’s corporation. Kirito eventually stops the experiment and rescues the remaining 300 SAO players, foiling Sugō’s plans. Before leaving ALO to see Asuna, Kayaba, who has uploaded his mind to the Internet using an experimental and destructively high-power version of NerveGear at the cost of his life, entrusts Kirito with The Seed – a package program designed to create virtual worlds. Kirito eventually reunites with Asuna in the real world and The Seed is released onto the Internet, reviving the VRMMO industry.