Mikoto, Sinon and Kirito have been living in the Underworld for 2 years now. Both Kirito and Mikoto, along with Eugeo, have become Elite Sword Masters each with their own Valet. Kirito has obtained his sword from the Gigas Cedar, the Night Sky Sword, while Mikoto has obtained the River Spirit Sword and dual wields it along with her Master Sword. Sinon in the meantime, has joined the Archery Corps.
When Mikoto's Valet, Stella, discovers a dark secret that nearly costs her life, Mikoto and Sinon decide to head out to investigate..
In the process they discover an entire armada of Viros swarming out of a strange portal near the goblin territories.
Meanwhile, in the real world, Accelerator and Asuna find themselves in a massive battle against the forces of God's Right Seat and secret forces from the Japanese Government, all of whom seem to be interested in Kazuto, Mikoto, and Shino..
And even stranger still, a dark figure seeks to find the Brain Burst Program, and develop something with it..
The world of Railgun Online is becoming larger still, and there's no telling where it might lead