11-year-old Kenichi Mitsuba is an average kid who goes to secondary school and struggles with his studies, he is very stubborn and is very lazy and therefore always ends up frustrating his parents and teacher. He loves to find an easy way of things. He befriends Hattori Kanzo, a ninja from the Iga Clan, and he becomes part of the Mitsuba family along with his brother, Shinzo and his ninja dog, Shishimaru. Hattori helps Kenichi with his problems, and constantly keeps an eye on him, as a good friend. The main antagonist Kemumaki, a Koga ninja and his ninja cat, Kagechiyo always troubles Kenichi, mainly because of their feud over one girl, Yumeko. Kenichi asks Hattori to take revenge as a recurring storyline in many episodes. Although Hattori is a good friend, Kenichi sometimes fights with Hattori due to misunderstandings created by Kemumaki. Sometimes Jippou, Togejirou and Tsubame help him.
El ninja más famoso de todos los tiempos se llama Hanzo Hattori y tiene 11 años, pesa 40 kilos y mide 140 cm.Viene a la ciudad donde vive Kenichi, un niño que no confia mucho en sí mismo y que Hattori elige para ser su amigo. Hattori junto a Shinzo su hermano y Shishimaru su perro ninja hacen cosas maravillosas como aprender técnicas nuevas de lucha, vivir aventuras alucinantes y además las habilidades de Ninja Hattori son infinitas.
忍者の里・伊賀からやって来たハットリくんがある日突然三葉家にホームステイ。同居する少年ケン一と、江戸時代とは違う現代の日本を体験。甲賀忍者ケムマキや弟シンゾウなど次つぎに面白いキャラクターが登場して、子供から大人まで楽しめる大人気コメディ。