Yoshikazu Miyano and his group return from a field trip ready to lead the Discipline Committee, but their plans take an unexpected turn when a student loses their keychain. Former chairman Masato Hanzawa creates the First Lost Item Detective Squad to track down the missing item. Shuumei Sasaki struggles to define his relationship with Miyano but joins the search party for the sake of staying productive. Despite their best efforts, the investigation hits a dead-end and the team must search every corner to find the keychain as more items go missing. It's crucial to solve the mystery before it's too late.