Natsume Tomoharu is an ordinary high school student in every respect, save for one anomaly: he is trailed by the ghost of his best friend, Misao. Upon moving into his brother's former residence, Tomoharu anticipates resuming his routine existence, only to find a mysterious, locked briefcase one day, devoid of any instructions. Initially intending to store it away, he soon finds his home overrun by various factions seeking the briefcase. Despite remaining unaware of the briefcase's purpose, Tomoharu and Misao strive to flee with it. Subsequently, Tomoharu endeavors to uncover the secrets surrounding the briefcase, its ties to Misao, and the reason behind its world-altering power.