I have to admit that when I watched the first episode, I was highly confused. The anime switches between the school life of Mato Kuroi and her friends and dark scenes of a girl named, Black Rock shooter, who is continually fighting. These scenes are not entirely explained in the first episode, so i felt compelled to watch more. Even if only to find out just what the hell was going on.

This is where the anime starts taking you on a rollercoaster by explaining that the darker world is a parallel to the current one. Mato's pain from her experiences in reality are being taken by her counterpart, Black Rock Shooter.

In this darker world, Black Rock Shooter is constantly fighting through that pain. And by fighting, I mean fighting, literally. The pain caused by Mato's friends and experiences in reality manifests as her friends counterparts in the dark world. And to free her friends from their pain, Black Rock Shooter must defeat them. 

In the end, Mato gets trapped within Black Rock Shooter, and must save herself from her own pain by defeating Black Rock Shooter. 

I started out thinking that both the girls in reality and the girls in the darker world were one and the same, but they are merely shadows of each other. One exists in reality, while the other exists merely to help the other. When the girls in the darker world become self aware, all hell breaks loose.

The anime was entertaining to watch, but confusing as to the concept of there being a counterpart to each person that would take your pain and fight through it. The anime is short as its only 8 episodes, plus an OVA, but I highly recommend watching it. 

See how deep this anime is for yourself!