Conan and detective boys go to read at Teitan elementary school's library. Kobayashi-sensei finds an old leather wallet hidden in a gab between the bookshelfs. There was a paper inside that wallet. Conan recognizes that wallet... 10 years ago, when Shinichi and Ran were at grade 1, they went to the school's library on a night with full moon, in order to confirm a rumor about ghosts appearing there. A mysterious man handed Shinichi that leather wallet, which had a paper with a message challenging them to solve a code.