Sword art online is a Virtual reality MMO. Initially, you start as the protagonist, who gets stuck inside of Aincrad, a 100 floor castle. The only way back to real life is by clearing all 100 floors. Hollow fragment starts on the 75th floor after defeating heathcliff. The game glitches out and as a result you get teleported into a hidden area known as hollow area. Here, you meet a female solo player in the woods named Philia, who just can’t seem to leave.