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Detective Conan OVA 03: Conan and Heiji and the Vanished Boy Description:

Plot Summary: Conan and Ran are visiting Heiji and Kazuha in Osaka but while they are touring around they come across a group of three kids that look like Ayumi, Mitsuhiko, and Genta. The group are looking for their missing friend and Conan agrees to help. You can also watch detective conan online from this beautiful detective conan site
Detective Conan OVA 03: Conan and Heiji and the Vanished Boy Episodes: