It has been a full year since the end of the Sword Art Online incident and ten months since the end of the ALfheim Online incident. Although things had returned to normal for some time, mysterious incidents occur in another VRMMO game, called Gun Gale Online (GGO), when in two instances, a player in the game is shot by a mysterious person in a mantle who calls himself "Death Gun" and the shot player mysteriously disconnects after a few seconds, only for their real body to be found dead days later. After the second such incident, Kirigaya Kazuto is asked by a member of the VR Crimes Division, Kikuoka Seijirou, to investigate the rumours that the incidents in-game are related to the deaths of the players in the real world.
After hearing that Death Gun has a requirement for his targets: the people he supposedly kills are all top players, Kazuto agrees to convert his ALfheim Online avatar into Gun Gale Online using The Seed's character conversion feature to enter the most well known tournament in the game, the Bullet of Bullets, to draw the attention of the mysterious Death Gun.
(Source: Sword Art Online Wikia)