Anonymous

Kousuke lives with his mother, Sakiko, and younger sister, Izumi, peacefully. But Izumi starts to withdraw at home, and they decide to move to another city. There are girls he knows at school where they transfer, Natsuki and Yuka, whom Kousuke met at the beach of his hometown last summer. But Natsuki seems to hate Kousuke. He doesn't know why, and can't understand her. One day, he receives an email from "Anonymous" saying, "I know your secret." He doesn't know what it is, but the email comes to his cell phone every day. It seems the sender enjoys seeing him being confused. He thinks something happened during the summer when he met them. He starts to investigate what happened on that day, even though he has a bad premonition...