A year has passed since Sword Art Online (SAO) was cleared. Kirito is asked by Kikuoka to investigate Gun Gale Online, specifically Death Gun, a weapon which seemed to link deaths in the virtual reality world to those in the real world.
Upon entering the game, Kirito meets Sinon who teaches him the ropes of the game. They both enter the Gun Gale Online tournament, Bullet of Bullets. Kirito displays his amazing skills although he is only a newcomer to the game. Later, Kirito finds out that the mysterious deaths are linked to a red guild named Laughing Coffin in SAO.
Summary taken from: MyAnimeList