When the Sanyo family welcomes a surprising new lodger, Hattori, a young ninja boy from the Iga mountains, their mundane life takes an adventurous turn. Hattori, along with his brother Shinzo, the ninja dog Shishimaru, and Kenichi's girlfriend Yumeko, joins forces with Kenichi as they navigate the challenges of attending a regular school in secrecy. However, their harmonious routine faces disruption from the rival Koga ninja, Kemumaki, and his sinister companion, Kagechiyo the Shadow-cat, adding a layer of suspense to their escapades.