Conan and his friends, along with assistant homeroom teacher Wakasa, discover a skeleton in the old storage shed at the school. The victim died approximately ten years earlier. Ten years ago, a group of four thieves broke into the home of a wealthy man and fled with 200 million yen worth of gold ingots. That same night, a group of suspicious figures were seen entering Teitan Elementary School. Conan and his friends attempt to crack the code written on the headband found in the skeleton's hand. Conan realizes the class schedule Ayumi found in the storage shed is the key to cracking the code.

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.