The eponymous protagonist is a naive young ninja from the Iga province called Kagemaru (absolute shadow), nicknamed Kabamaru (hippo's mouth) for his insatiable appetite.

The plot deals mainly with Kabamaru adjusting to an urban lifestyle after spending a childhood in the mountains, him developing feelings for his guardian's granddaughter, and how he finds himself caught up in the rivalry between two private schools.

Kin'gyoku School and Ōgyoku School have been bitter rivals for almost half a century, largely due to the rivalry between the wealthy and influential families of Shuu Maejima (前島秀, Maejima Shū), head of the Ōgyoku student council, and Shizune Mejiro (目白沈寝, Mejiro Shizune), a student council member at Kin'gyoku.

Maejima extends his control over the student councils of other schools, forming secret "assault squads" there and enlisting members of criminal gangs to do his dirty work.

Just as everyone thinks Hayate has drowned, he emerges from the water, along with grandfather Saizō, who faked his death to introduce Kabamaru to the second stage of ninja training and the responsibilities of an adult.

One main reason for this was the low quality of English-based dubbing often called "so bad that it's actually good", but it nevertheless impacted the growth of anime in the country.

It starred Hikaru Kurosaki as Kabamaru, Kumiko Takeda as Mai, Sanada Hiroyuki as Shizune and Sonny Chiba as Saizō.