Wataru Minakami is a top student who failed his high school entrance exam due to a computer glitch. He later discovers that he has been accepted to Stargazer Hill Academy, located on the mysterious Promised Island. At his father's request, Wataru is taken to the island, and before he can settle in, a dozen cute and charming girls flock to him, claiming to be his younger sisters. As Wataru grows closer to his newfound siblings, a deeper mystery regarding why they were sent to the island comes to light.