Detective Conan OVA 03: Conan and Heiji and the Vanished Boy
Directed by Yasuichiro YamamotoA successful detective gets injected with a virus which makes him a ten year old boy. But as his brain and mind are similar as before he helps his girlfriend's father (an unsuccessful detective) to solve cases without getting known to anyone.
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Cast of Detective Conan OVA 03: Conan and Heiji and the Vanished Boy