After a field trip, Yoshikazu Miyano and his friends return to find themselves poised to lead the Discipline Committee. However, a lost keychain sparks an unexpected turn of events orchestrated by the former chairman, Masato Hanzawa, leading to the formation of the First Lost Item Detective Squad. Meanwhile, Shuumei Sasaki grapples with defining his evolving relationship with Miyano and decides to assist in the search. Despite Sasaki's involvement, the search proves fruitless, prompting the group to intensify their efforts. As they delve deeper, they unravel a larger mystery of numerous missing items, compelling them to solve the puzzle swiftly.