In 1929 Japan, the practice of being a ninja is banned by a corrupt government officials who feared opposition in their quest for power. Unfortunately, 9-year-old Kenji has just fallen in love with the craft after watching his grandfather train, which leads to the boy witnessing the death of his grandfather by those same government officials. At this moment, Kenji, with the help of his grandfather's friend, decides to devote his life to secretly training in the ways of the ninja and take his revenge on those who killed his grandfather.