PlotIn the year 2022, the Virtual Reality Massively Multiplayer Online Role-Playing Game (VRMMORPG), Sword Art Online (SAO), is released. 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, all the players log in for the first time, and subsequently discover that they are unable to log out. They are then informed by Kayaba Akihiko, the creator of SAO, 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. The story follows Kirito, 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 in SAO, they confront and defeat him, freeing themselves and the other players from the game.
Upon returning to the real world, Kirito learns that Asuna and 300 other SAO players have still not awakened yet. Following a clue about Asuna's whereabouts in another VRMMORPG called Alfheim Online (ALO), Kirito also logs in there. Helped by his sister Suguha, known as Leafa in the game, he learns that the trapped players in ALO are part of a plan devised by Nobuyuki Sugō to perform illegal experiments on their minds to put them under his control, including Asuna, whom he intends to marry in the real world in order to take over her family's company. After Kirito stops Nobuyuki's plans, he finally reunites with Asuna in the real world.
Soon after, Kirito plays another game called Gun Gale Online (GGO) to investigate the mysterious connection between it and deaths occurring in the real world. Assisted by a female player he meets in the game called Sinon, he identifies and exposes the culprits, who include some former members of a murderous guild he had previously encountered in SAO.
Kirito is later recruited to assist in the development of a state-of-the-art game, UnderWorld (UW), which has an interface that is far more realistic and complex than the previous games he had played. In UW, the flow of time proceeds a thousand times faster than in the real world. However, he ends up falling into a trap set by one of the murderers from GGO and wakes up inside the game, unable to log out, with his real self left in a comatose state. Eventually he starts to question if he is the real Kirito or an artificial intelligence modeled after him.
The light novel series spans several virtual MMORPGs, not exclusively the eponymous world of Sword Art Online.
- Sword Art Online (ソードアート・オンライン Sōdo Āto Onrain )
- The first virtual game world, as well as the setting of the first arc of the story, known as SAO for short. The world takes the form of a giant floating castle called Aincrad, with 100 floors in it. Each floor has a medieval-themed setting and a dungeon with a boss, which has to be defeated before players can advance to the next higher floor. Like most RPGs, it implements a level-based system. However, the game is altered in a manner in which players are unable to log out, and if players die in-game, their real life body dies too.
- Alfheim Online (アルヴヘイム・オンライン Aruvuheimu Onrain )
- The setting for the second arc of the story, known as ALO for short. All players in the game have wings and are capable of flight. It is a large world, divided into separate 'homelands' for each of its fairy races. In Alfheim's center is a very large tree called the World Tree, and the goal of the game is to reach the top. It implements a skill-based system with players increasing their stats by developing both their combat and non-combat skills. Aincrad, the castle of the first game, is later accessible to ALO players as well.
- Gun Gale Online (ガンゲイル・オンライン Gan Geiru Onrain )
- The setting for the third arc of the story, known as GGO for short. It is a virtual game world with a main focus on guns, although melee weapons like lightsabers and knives also exist. From all the games it is the most competitive one as the money earned there can be exchanged for currency used in the real world, drawing high-tier professional players to make a living from it.
- The setting for the fourth arc of the story. According to Kirito, it is graphically the most realistic of all VRMMOs to date. The flow of time in UW is variable and can be much faster relative to the real world's. Even so, it seems that even Kirito was misled as to the actual rate of UW's time flow, so it is still unknown. However, in the as of yet canon "There is but one ultimate way" side story, the rate is said to be safely up to 1,000 times faster than that of the real world, just like Accel World's Brain Burst.