In 1995, all 800 habitants of the isolated Kamikakushi Island vanished in one night, all but a single girl who lost her memory. The cause of the disappearances was never identified.
Four years later, that girl, Mari, transfers to a girl's dormitory school, Kaihou Gakuen, leaving behind her grandmother who had been her guardian ever since the incident. There, she meets Hagino.
Hagino is the school idol and completes her student council responsibilities flawlessly. The moment she touches Mari’s hand, however, Hagino freaks out, straddling atop Mari, hands aroundher throat. Mari is afraid of and angry at Hagino, who pretends as if nothing had happened while in front of other students, but at the same time, somehow, she is attracted to Hagino too.
In reality, Hagino is actually an alien who was sent to Earth in order to reconnoiter prior to an invasion. The event on Kamikakushi Island occurred when the 200-member crew of a battleship, the Blue, whose captain was Hagino and the island's inhabitants had killed each other. The Blue was seriously damaged and hidden. Hagino had since blended into human society and continued her reconnaissance.
Within her school life, while fighting with fellow students, Mari begins to come out of her shell. Hagino has been living a double life, but her encounter with Mari makes her face human beings, and gradually she begins to have doubts about her mission. While the invasion and war draws closer unnoticed, the girls begin to understand each other.
