The ordinary Sanyo family receives an unexpected surprise in the form of a new lodger: Hattori, a ninja boy who has descended from the Iga mountains to attend a regular school. After befriending Kenichi Sanyo, who is his age, Hattori begins attending school undercover, sometimes accompanied by his brother Shinzo, the ninja dog Shishimaru, and Kenichi's girlfriend Yumeko. The group is also threatened by the rival Koga ninja Kemumaki and his evil sidekick, the shadow-cat Kagechiyo.