The Ozunu Clan, led by the ruthless Lord Ozunu, trains orphans from around the world to become the ultimate ninja assassins. Raizo is one of the orphans. Their training is extremely brutal and tough, especially for him, because he is to be the next successor of the clan. The only kindness he ever feels is from a young kunoichi named Kiriko, with whom he eventually develops a romantic bond. As time goes by, Kiriko becomes disenchanted with the Ozunu's routine and decides to abandon it. One rainy night, Kiriko climbs a wall to escape and encourages Raizo to join her, but he chooses to stay. Branded as a traitor, Kiriko is captured and later executed in front of Raizo by their elder ninja brother Takeshi, who impales her through the heart.