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.