Ishigami, who succeeded in inviting Tsubame on a Culture Festival date, browses around campus exhibits in an attempt to shorten the distance between the two. In the end, it didn't go as well as hoped. But when he tried to express his everyday feelings of gratitude for the upperclassman he feels indebted to, it caused an unexpected uproar. Shirogane was astonished after witnessing the unexpected event. Gripped by the fear that his secretly planned "Make Shinomiya Confess Plan" will be ruined, he sinks into despair after thinking too hard about every detail.

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...

Shroud in a cape and dressed in the guise of the phantom thief Arsène, Shirogane waited for Kaguya's arrival atop the clock tower roof overlooking Shuchiin. When asked why he would do such a thing, Shirogane answered that there was something he wanted to show Kaguya. The festival's afterparty has already begun as the sun set over this school and the students could be seen encircling and enjoying the bonfire. While the emotions that could not be expressed with words mingled between Shirogane and Kaguya's hearts... Shirogane carried out his ultra-romantic operation!!