The Juumonji Incident is a case featured in Volume 3 of the Kotenbu series and Episodes 12 to 17 of the Hyouka anime series.
Each year Kamiyama High School organizes it's cultural festival called "Kanya Festa". But this year a thief by the name of Juumonji begins stealing items from different clubs within the school, leaving behind a card and the festivals brochure.
Meanwhile the Classics Club have finished their anthology and intends to present it as their attraction for the festival. However, Mayaka Ibara made a mistake and ordered excessive copies. Eventually, Hōtarō Oreki gets convinced that by solving the Juumonji incident, he might be able to attract people to the Classics Club in order to sell all the copies of the Hyouka anthology.
Following the yearly activity of the Classics Club throughout its history in Kamiyama High School, current members Houtarou Oreki, Eru Chitanda, Mayaka Ibara and Satoshi Fukube organized the Hyouka anthology for the current year during summer vacation, where they wrote about Eru's uncle and his story as a member of the Classics Club.
Due to a mistake committed by Mayaka during the publishing process, the club is pressed to sell more copies of the anthology than anticipated. Each of the members did their best to make the cut: Houtarou stayed to sell copies from inside the clubroom and Eru and Satoshi spent their time joining events and asking for help while Mayaka occasionally visits after being preoccupied by her duties as a member of the Manga Club. During Eru's journey around the campus, she meets Kaho Jumonji, and soon, she finds out about the missing items throughout school, which the other members eventually discover in separate events.