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 in a mysterious place known as Promised Island. At his father's request, Wataru is whisked away 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.