A virtual reality gaming system known as the Nerve Gear is finally released to the public alongside the world's first virtual reality game called Sword Art Online (SAO). Kirito, one of the game's beta-testers, logs-in to Aincrad, the world of SAO and quickly befriends novice player Klein, to whom he teaches the basics of the game. After a few hours, Kirito and Klein discover that they cannot log-out. Akihiko Kayaba, the game's creator and employee of the computer corporation Argus, announces to the players that he intentionally removed the log-out option and that all players will die if their Nerve Gear is forcefully removed or their in-game health bar is depleted to zero. He informs them that he created this game as a way to become God over his own dominion. The only way to escape is to clear all 100 floors of Aincrad.
One month since the start of the fun-turned-deadly game, 2,000 players have already died, and the first floor has yet to be cleared. Kirito attends a meeting organized by fellow beta-tester Diabel, who announces to the attendants that his party has found Illfang the Kobold Lord, the boss of the first floor. Parties are formed, and Kirito teams up with a young girl named Asuna.
After saving the Moonlit Black Cats guild from a monster attack, their leader, Keita, thanks Kirito and asks him to join their guild. Kirito accepts the invitation, but hides the fact that his level is twice the average level of the Moonlit Black Cats guild. Kirito bonds with his new friends, especially Sachi, who confides in Kirito that she is afraid to die. Kirito assures her that he will protect her, and they will be free one day.
A beast tamer named Silica quits a party after arguing with their leader, Rosalia. Later, Silica is ambushed by a group of monsters while making her way alone through the forest, and her dragon familiar, Pina, is killed while trying to protect her. Kirito arrives and saves Silica. He tells her of an item at the Hill of Memories on the 47th floor, which can revive her familiar within three days of its death, and offers to accompany her there.
Asuna offers to buy dinner for Kirito, but their meal is interrupted by a scream. They rush out of the restaurant to find a player, Kains, hanging outside a church, impaled with a spear. He dies immediately after. Fearing there is an unknown player versus player skill, they began to investigate the murder.
Kirito and Asuna continue to investigate the murder of Kains and Yolko, who were members of a disbanded guild called Golden Apple. Kirito wonders how it is possible to kill someone in a safe zone. Schmit, another former member of Golden Apple, declares that the killer is the ghost of Griselda, their deceased leader. After watching a dropped sandwich vanish, Kirito deduces how Yolko and Kains died, and later, who murdered Griselda.
Yoshitsugu Matsuoka
Kirito
Haruka Tomatsu
Asuna
Kanae Itō
Yui
Miyuki Sawashiro
Sinon
Ayana Taketatsu
Leafa / Suguha Kirigaya