Fifty years ago an enormous Tower materialized in the middle of Tokyo. The city was destroyed and the majority of people died. The military tried to damage the tower but their weapons didn't even leave a scratch.

The military started to explore the outside of the Tower but all they could find was a gate. The military sent the first wave of people inside to explore the Tower but when they come back barely one-tenth of them survived.

Inside the tower, a new world appeared in front of their eyes. The difference was that this world was extremely dangerous because powerful beasts were everywhere.