Conan, the Detective Boys, and their assistant homeroom teacher Wakasa discover a ten year old skeleton in the basement of the old school storage shed. They begin attempting to crack the code written on the headband the skeleton was holding in its hand. The class schedule Ayumi found in the storage shed is the key to the code, which is related to a ten year old case in which four thieves stole 200 million yen worth of gold ingots from a wealthy man's home. The skeleton belonged to the boss of the thieves. Conan cracks the code and comes up with a plan to catch the other three thieves who are coming for their shares.

Mouri Kogoro received a letter from a mysterious person called Kurataya Komakichi saying he wanted to give him his family treasure. So, Ran and Conan decided to accompany him to Yamaguchi.