Raised in the mountains, Kanzo Hattori, aka Nin, is one of the last descendants of the Iga ninja clan. Upon reaching manhood, his father, the great master Hattori, orders him to fulfil his destiny by going to Tokyo. Upon his arrival, his life is saved by young Kenichi. Following the ninja code, Nin swears him allegiance, and must serve his young master unseen by the locals. Immersed in the urban jungle of the 21st century metropolis, Nin discovers that the Koga clan, his ancestral rival, has broadly infiltrated society and is ready to act.