Sister Princess is the story of how Wataru found that he had a number of sisters and how he learned to care for them. He was sent to "Sister Island" to meet the sisters. He had thirteen sisters. At first he is very reluctant to go and wants to run away back to Tokyo. Soon, though, after numerous tries to run away to Tokyo, he starts to feel comfortable and realizes how much he loves his sisters. They are all very different from each other. Little does he know, however, that one of his "siblings" is actually a "spy" sent from Wataru's friend to bring him back to Tokyo.