The Jumonji Affair

Classic Lit Club finds a way to sell all "Hyouka" anthology copies. To accomplish this, Houtarou needs to catch mysterious "Juumonji" thief who is responsible for various thefts during Kanya Festival.

Plot
Houtarou is eating a lunch made by his sister, Tomoe Oreki, only to comment how it's quite "ethnical", even though she doesn't usually cook for him. Satoshi, Mayaka and Eru return from the cooking competition, and Houtarou congratulates them. Eru thanks Houtarou for the flour he gave them. Houtarou says that he didn't do anything significant, and Satoshi comments that he didn't expect that from ever-lazy Houtarou. In response, Houtarou says that he was driven by the festival mood, and therefore this wouldn't happen again.

Eru asks Houtarou to take a look at the note left by mysterious "Juumonji" thief after he had stolen ladle from Cooking Club. Mayaka comments that during cooking contest ladle was missing only at their station. Satoshi adds that Go Club and A Capella Club also had experienced something similar, and Eru saying that the same had happened to the Fortune Telling Club. Houtarou doesn't want to investigate, so he responds that this thief must be really bored to do something so bothersome. In order to change conversation topic, Houtarou make a remark about how spicy his meal is, but he fails as Eru stares at him with her "I'm curious!" look. Houtarou is dumbfounded, being unable to resist Eru's gaze, which reflects her curiosity firing up.

Eru starts asking why this thief is doing such things to get in the Kanya Festival's way. She also asks why they name themselves "Juumonji" and why they keep taking item after item from different clubs. Houtarou tries to stop Eru and tells her that they need to focus on "Hyouka" sales, but Satoshi interrupts him. He tells Houtarou that advertising they have done to sell "Hyouka" may not be enough to sell all the anthologies. Mayaka agrees, saying that probably they need to do something more flashy. Satoshi adds that they can solve this "Juumonji" mystery and use it to promote the anthology by announcing it during tomorrow's school-wide radio broadcast.

Classic Lit Club and the "Juumonji" Incident's Relation case
At this year's Kanya Festival, a thief has been stealing items from a few clubs in Kamiyama High and left a note after they committed the crime, but the items they stole are just ordinary things with not much value at all. The clubs that have become victim only noticed that something was stolen, but never saw who did it or even know when it happened. Eru and Satoshi found out from their acquaintances about this incident that happened to the other clubs like the A Capella Club lost a drink, Go Club lost stone pieces and Fortune Telling Club lost a tarot card. And Eru found one of the notes from the culprit named Juumonji which is found after the end of Wild Fire, a cooking competition hosted by his latest victim, the cooking club who lost a ladle and brought the note to Houtarou. Houtarou was not interested with it at first, but in the end he started discussing about the incident with the others since they decided to solve the mystery of the incident and figure out Juumoji's true identity as a chance to promote themselves. Houtarou, Satoshi and Mayaka noticed that Juumoji's Modus Operandi is based on the Japanese alphabet similar to Agatha Christie's The A.B.C. Murders. By asking other people, Satoshi found out that the Gardening Club also got targeted and lost a water gun. And since in the story the victims were murdered in alphabetical order of their names and town they were murdered at, they compared each club and their lost items and confirmed that the pattern is very identical. Houtarou also suggested that the signature Juumonji in the note is not a name but rather the phrase Juumoji that means "Ten Letters". In other words, Juumoji would keep stealing items from other clubs in the order of the first ten of ' (A,I,U,E,O,Ka,Ki,Ku,Ke,Ko''') and the Classic Lit Club are bound to get involved since they could potentially be their last victims which gives them a chance to promote themselves.

Phantom Thief Juumonji case
After figuring out his Modus Operandi and next moves, Eru went to the Wall Newspaper Club to warn them and found out they lost a utility knife. Satoshi also gathers info from other clubs and found out A Capella club lost an Apple Juice. After that, he decided to catch Juumoji in the act by going to the Magic Club's magic show, which is going to be their next target. But, Juumoji beat him to the punch and stole a candle from the Magic Club. Houtarou in the other hand asked Mayaka about The A.B.C. Murder. He asked the motives behind the incident of the story. From there, she asked him if the incident could have the same motives just like in The A.B.C. Murders. Houtarou deduced that Juumoji's Modus Operandi is a pattern anyone could think of and that would make them to be very predictable. But from how consistent they are with this, he believed there must be a reason behind the incident.