In a world where, by the powers of Divine Law, people cannot injure one another, war is waged through proxy with monsters and magical constructs. A new revolutionary armament enters the fray: a human soul from another world merged with the body of a golem, creating a powerful ReConstruct. Though ReConstructs are protected by the Divine Law, they are not bound by it, making them seemingly invincible warriors.

A man from Earth is transported to the world of Mene, and his soul placed into the body of a ReConstruct. Thus became Number BS-8, and nicknamed Bastion by his creator's apprentice, Sybille. At first Bastion was promised by his creator, Master Croghan, that he would only face against monsters, not humans. The truth of war, however, soon crept up on him. Now, the struggle between duty and morality became a choice he could not avoid.

Will Bastion accept his purpose as a weapon of war, or with Sybille's help, betray his intended purpose, and seek freedom in the land of Mene?