The Juumonji Incident (十文字事件 Jūmonji Jiken) is the fifteenth episode of the anime series.
Hōtarō 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 returns from the cooking competition as Hōtarō congratulates them. Eru gives her thanks to Hōtarō, for without the flour he gave the group, they would not have won the cooking contest. As Hōtarō is in denial that he helped much, Satoshi comments on how Hōtarō's earlier action was unexpected, due to Hōtarō's lazy way of living. As an excuse as to why he gave the group the flour, Hōtarō said he was driven by the festival mood, therefore he will not do it again.
Changing the subject, Eru presents one of the many notes, given out by the thief Juumonji, asking for Hōtarō to take a look at it. The note is the one the cooking club found, where their laddle was missing. Mayaka makes note that the laddle for their station at the cooking competition was the only one missing. Satoshi also states that the Go club and A Capella club has experienced something similar, with Eru saying that the same has happened to the Fortune Telling club. To avoid this shower of comments, Hōtarō comments how bored this thief must have been to do something so bothersome. In order to change the entire topic, Hōtarō attempts to comment how spicy his meal is, but fails when Eru stares with her "curiosity" eyes. Hōtarō is dumbfounded as he is now locked in Eru's gaze, due to her curiosity firing up.
Eru goes on to asking why this thief is doing such things to get in the festival's way. She also asks why they name themselves "Juumonji" and why they keep taking items after item from separate locations. To put Eru's curiosity to a stop, Hōtarō attempts to tell her that they need to focus on the Hyouka sales, but Satoshi cuts in. He tells Hōtarō that advertising in events may not be enough to sell all the anthologies. Mayaka agrees, saying that they might need to do something more flashy. Satoshi follows, that they could use this mystery to promote the anthology. to go even farther, they might as well announce this during tomorrow's school-wide radio broadcast. They could sell all the copies of the anthology.