The everyday Sanyo family gets an unexpected surprise in the shape of a new lodger-Hattori is a ninja boy who has come down from the Iga mountains to attend normal school. Befriending Kenichi Sanyo, who is the same age as he, Hattori starts attending school undercover, occasionally accompanied by his brother, Shinzo, Shishimaru the ninja dog, and Kenichi's friend, Yumeko. The gang is also threatened by the rival Koga ninja, Amara, and his evil sidekick, Keo the Shadow-cat.

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