One day Mori Kogoro's private detective agency receives a blackmail call. It turns out that the caller has dialed the wrong number. Conan is able to identify the voice on the phone, it's similar to that of his old friend Heiji, who's also a high school detective but in Osaka. They join once again forces to solve the case and find the jewel which was stolen.

But they are not the only ones who are after the jewel. The notorious master thief Kaitou Kid has taken an interest in this piece, which is worth 100,000,000 Yen, as well and now it's up to Heiji and Conan to be the first to find the culprit and acquire the jewel.

While investigating a murder, suspicions fall on Conan for being too smart so Conan plays dumb for the rest of the day, much to the Detective Boys dismay.