Hotaro challenges Eru to come up with a random mystery for him to solve in order to prove that a theory can be created out of anything and that Eru can't rely on Hotaro about everything she ponders. After it's announced on the school's intercom, Eru settles on having Hotaro figure out why students who went to a local stationery store are being called into the staff room. Much to Hotaro's discomfort, Eru takes the "game" too seriously, and she becomes dissatisfied with the results. Remembering a news flyer posted on a bulletin board in the school, Hotaro deduces that students are being called to the staff room in order to help authorities find the source of a counterfeit 10,000 yen bill given to the clerks at the stationery store. The theory seems perfect, and Eru is impressed and speechless. However, both she and Hotaro forget why they came up with the game in the first place. The next morning, Hotaro sees in the newspaper that the culprits of the counterfeiting scheme have been apprehended, confirming the accuracy of Hotaro's mock theory.