Conan, Heiji, and the Vanished Boy

Situation
Heiji invites Conan and Ran Mouri to Osaka again. Along with Kazuha, they show them Osaka's major sights. In the middle of the sightseeing they run into three kids named Mayumi, Kenta, and Mikihiko who are waiting for their friend Satoshi. Since Satoshi is late, they ask Heiji to help locate Satoshi. The kids say that Satoshi is at his secret base which is only revealed through Satoshi's riddle. The kids explain Satoshi found some jewels at his secret base and Heiji reveals Satoshi's secret base which is jewelry robbers new hiding place.

Resolution
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After figuring out Satoshi's riddle, they are able to find Satoshi at his old house and arrest the robbers.
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Trivia

 * This OVA features a brief reference to movie 7 in the form of Heishiro Bizen.