One week after enrolling in Shuchiin Academy as a special scholarship student, Shirogane had already regretted that he had come to a school he should not have. The students around him were elites born into famous families. Shirogane, who is from a modest household and enrolled from a school outside of Shuchiin's track, couldn't even find a friend to eat with, so he quietly kills time away from the cafeteria. Just as Shirogane's miserable emotions explode, an upperclassman with a student cap appears in front of him. A shining golden fourragere lays across his chest as proof of student council president. And he says that he's come to recruit Shirogane into the student council.

The moment the culture festival reached its finale, the jewel object made by Shirogane was stolen by a phantom thief who calls himself Arsène. Kaguya and the others headed to the roof where the phantom thief keeps appearing, but there was no one there. Fujiwara, who transformed into a genius detective, struggles to solve the mystery behind the calling card and uncover the thief's true identity. But Kaguya, left behind at the scene of the crime, had begun to figure out the criminal. The problem is the location. Kaguya traced the phantom thief's line of thinking and headed to where he is now...