In 2115, the country known formerly as Japan has been split into two parallel worlds: the Frontier S (Stratos) and the 17th Far East Imperial City Management District. Frontier S is an outlaw universe in which freedom runs rampant, while the 17th Far East Imperial City Management District is a totalitarian universe completely bereft of freedom and is under constant surveillance. When the two universes are starting to fuse into one, the government of each worlds initiates the Operation Stratos, a world-scale protocol wherein a handpicked group of gunslingers, mercenaries, and special individuals, each from the two universes, will be sent in the year 2015 to alter the past by eliminating the other side until only one group survives, erasing a parallel universe in the process.
from Wikipedia'