After returning from a field trip, Yoshikazu Miyano—along with his friends—is ready to take over the Discipline Committee. But when a student loses their keychain, the now former chairman Masato Hanzawa uses this opportunity to create the First Lost Item Detective Squad in order to find the misplaced belonging. Amidst all this, Shuumei Sasaki is trying to determine how to define his relationship with Miyano going forward. Hoping to be helpful to Miyano, he joins the search party. Yet, even with Sasaki on board, the investigation remains unsuccessful. The group must continue to look everywhere and solve the mystery surrounding the keychain soon—as they discover that many more items have gone missing.