A gate to another world opens in Mikado City, and monsters called Neighbors emerge. It's up to Border Defense agent Osamu Mikumo to fight them.

One day, while playing the online game Elder Tales, 30,000 players suddenly find themselves trapped in another world. There, eight-year veteran gamer Shiroe also gets left behind. The trapped players are still alive, but they remain in combat with the monsters. The players don't understand what has happened to them, and they flee to Akiba, the largest city in Tokyo, where they are thrown into chaos. Once proud of his loner lifestyle, Shiroe forms a guild called Log Horizon with his old friend Naotsugu, female assassin Akatsuki and others.