From the top of the metropolis S1-9, the dominant Han rules with fists of iron and flesh. Beneath the city's rotten foundations, a vengeful warrior embarks on a journey to assassinate him. 5 years before the events of Slave Zero, Slave Zero X breathes new life into a world where horrific, living machines known as Slaves are poised to become the latest tools of war in humanity's long and bloody history.

A secret group of warriors, known as The Guardians, hopes to stop the unleashing of these bio-machines upon the world, but a swordsman from their ranks has a different idea: to use the enemy's own weapon against him. Merging with a stolen prototype Slave Unit, Shou will become a raging Devil seeking to kill a false God.