The title character is a gullible young ninja from Iga province named Kagemaro (meaning 'absolute shadow'), who goes by the nickname Kabamaru ('hippo's mouth') due to his bottomless appetite. Following the passing of his rigid grandfather and ninja instructor, Kabamaru relocates to Tokyo with one of his grandfather's contacts. The story primarily focuses on Kabamaru's adaptation to city life after growing up in the mountains, as well as his entanglement in the rivalry between two private schools.