Kain is a story about a blue eyed boy wandering the countryside. He happens to be in a town controlled by "kidoushi". These "kidoushis" are people from the newborn country of Ren, who have a machine in their body, giving them power equal to an army. The power of the kidoushi is determined by the amount the gigai (the machine) take up in their body.
Volume 2: Kain (pilot)
Volume 3: Forest