Kogoro receives a letter from a mysterious person named Kurataya Komakichi claiming he wants to give him his family treasure, so he heads to Yamaguchi prefecture with Conan and Ran, but he's mistaken for a robber's accomplice and is arrested. Komakichi is the name of a robber who shot owner of Japanese-style cafe Takarada in the arm and stole millions of yen worth of Keicho gold coins. Kogoro is released after news site reporter Asuka clears up the misunderstanding. Kogoro, Conan, and Ran visit Hagi in their investigation into the case, but someone is trying to kill them.
