Detective Conan OVA 06: Follow the Vanished Diamond! Conan & Heiji vs. Kid!
One day Mori Kogoro's private dective 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 Yen, as well and now it's up to Heiji and Conan to be the first to find the culprit and acquire the jewel.