Natsume Tomoharu is an ordinary high school student in every respect, save for one detail: he is trailed by the ghost of his best friend, Misao. Upon moving into his brother's former home, Tomoharu anticipates resuming his routine life, but one day finds himself in possession of a mysterious, locked briefcase with no accompanying instructions. Initially intending to store it away, he soon finds his house overrun by various groups pursuing the briefcase. Still unaware of the briefcase's purpose, Tomoharu and Misao attempt to flee with it. From that point forward, Tomoharu seeks to uncover the secrets surrounding the briefcase, its ties to Misao, and the reason it holds the power to alter the world.