Before she left her rural village to go to school in the big city, 6-year-old Shinobu told her friend Saizou to grow up strong so he could someday come and protect her. Now 16, Shinobu's enjoying life at the Tokyo Private Imperial Academy. But when a male exchange student comes to the school, it turns out to be none other than Saizou! Although his appearance hasn't changed much, he is now a skilled ninja. And his arrival couldn't be more timely: Someone is out to get Shinobu, but no one knows who or why. Saizou wants to be more than a protector to his old friend—but he may have a rival.