With the club recruitment week happening at First High School, Honoka and Shizuku, who are all-round outstanding students, are asked by various clubs to join. When they find themselves mobbed by club members, Eimi comes to the rescue, and the three of them become fast friends. One the way home from school, Honoka and her friends witness Tatsuya, who recently became a member of the Disciplinary Committee, being attacked by someone. Eimi, who is driven by her interest in this incident and in Tatsuya, decides to investigate who the attacker is. Honoka and Shizuku get pulled into Eimi’s investigation and the three girls end up forming the Girls’ Detective Club.

The Nine Schools Competition begins, and Shizuku goes head-to-head with Shiori in the Speed Shooting tournament. Shiori utilizes the “Arithmetic Chain” magic sequence which allows her to perform mathematical calculations faster than a supercomputer. Shiori left her own broken family to join Airi’s team, and her determination to win drives Shizuku into a corner. Honoka and her friends nervously watch the match, but Miyuki says, “Who do you think Shizuku’s engineer is?” with a confident smile. Just as she has predicted, Tatsuya has a secret tactic up his sleeve to bring victory to Shizuku.