Kagemori Mamoru is a dowdy boy whose hair is disheveled and wearing thick glasses. But actually he is the son of a ninja family. They have protected their neighbor, Konnyaku family, secretly for 400 years. Mamori has also protected the daughter, Konnyaku Yuuna, since they were in the kindergarten. When Yuuna is in trouble, Mamoru puts on ninja suit quickly, and he turns into a competent ninja to help her.
cont. from above summary..
Because of Yuuna's unawareness, Mamoru often goes through many depths to save her as her ninja protector, but the series is basically about how they make new friendships, have new hardships, and strengthen their relationship with each other, mostly without Yuuna's knowingness of this..