Like many high school students, Kagemori Mamoru is obsessed with a girl. Unlike most students, however, the main reason that Mamoru is so focused on Konnyaku Yuna is because his family has spent the last 400 years protecting her family from every kind of threat imaginable. (This is what happens when one of your ancestors swears a binding oath and you're a family of ninjas.) Unfortunately, Mamoru's job is made much more difficult by the fact that Yuna is incredibly clueless and accident prone, forcing him to work around the clock keeping his oblivious childhood friend from accidentally offing herself. And it's made doubly more difficult given that Mamoru has to perform his protecting duties in secret, so he has to pretend to be a lot less competent and skillful than he really is when not in full ninja garb. And just to make things absolutely impossible, the complexity level gets quintupled and escalates as the school inexplicably begins to fill with girls with various ninja and magical-type skills who seem to have their eyes on Mamoru himself! Study hall will never be the same as high school shenanigans collide with shurikens in Guardian Ninja Mamoru!