A year after SAO was cleared, an American company developed and released a game of their own based on the World Seed; a VRMMOFPS, Gun Gale Online (GGO).
In an in-game bar, players watch a livestreamed interview with the winner of an in-game tournament, XeXeed. To the annoyance of the bar patrons, XeXeed brags about how he won by tricking players into focusing on making Agility-based builds while he made a Strength-based build that offered him a decisive advantage.
One player makes quite a show of his grievances againse XeXeed. Cloaked in black and wearing a face-concealing voice-distorting mask, he declares that he will pass judgment upon XeXeed and fires his pistol at the screen. The other players laugh at what they believe to be a show of impotent rage, but the laughter stops when XeXeed suddenly clutches his chest and disappears from the livestream. The shooter than introduces himself and his weapon as "Death Gun".
A month later, Kazuto Kirigaya, aka Kirito, meets up with Asuna Yuuki for a date touring an Imperial palace. The two have a conversation about the nature of data, and how the differences between the real world and the virtual world come down to the amount of information present in each. Kazuto also discusses his plans for after graduation: he wants to help develop new Fulldive technology, and research augmented reality, so that one day, he could bring their AI foster daughter Yui into the real world.
Before going out with Asuna, however, Kazuto had met with a member of the SAO Incident Task Force, Seijiro Kikuoka, a man with whom he shared a long and untrusting history after he returned from SAO. Kikuoka told him about a series of mysterious deaths: two people had turned up dead while Fulldiving using Amusphere VR headsets, with each death ruled to be due to heart failure. What both of them had in common were that they were high-level players of Gun Gale Online. A witness described a player shooting at a monitor featuring the face of one of the players, known ingame as "XeXeed", who died mere moments later. The killer then introduced himself as "Death Gun".
At a loss for how someone was able to kill another person through a video game, Kikuoka asks for Kirito's help, offering to compensate him.
That night, while playing with his friends in ALO, Kirito makes a decision...
