Kirito (Kazuto Kirigaya), along with approximately six thousand other SAO players, successfully returned to the real world after clearing Sword Art Online. However, including Asuna, about three hundred players remain in a comatose state. SAO's operating company, "ARGUS," was also acquired by Asuna's father's company, "RECT." When Kirito visited the still comatose Asuna, he was confronted and belittled by Sugou Nobuyuki (the foster son of Asuna's father), leading to a sense of despair.
Later, Kirito learned of information suggesting that Asuna might be imprisoned in another VRMMORPG called "ALfheim Online." In order to rescue Asuna, who had not yet returned from SAO, Kirito logged into the suspicious VRMMO "ALfheim Online." Additionally, Kirito's sister, Suguha, known as Leafa in ALO, happened to be one of the players in the game. The two siblings unexpectedly encountered each other in the game without knowing each other's true identities.