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.

An order has been sent out to hunt down the existence of an Asklepios, a Greek demon god of medicine, whom the Church believes is practising unorthodox methods. A member of armed force group assigned to this mission by the name of Luca encounters two kids who saved him from an injury he faced during a brawl. Who are these kids and can they help out Luca with his personal reason for doing this assignment? One-shot turned series in September 2008.