When a strange group of men attempt to kidnap Atsuko Fukune, a dancer at a club, she seeks the help of Mamiya Mugen, a famous detective - who also happens to be a child. When it turns out that other girls have gone missing, Mamiya, with the help of his butler Alucard, springs into action to solve the mystery.
 
(Source: Anime News Network)

This is the second iteration of the Mugen Shinshi franchise and is generally referred to as Mugen Shinshi Bouken Katsugeki-hen.
 
Note: In 2007, volume 1 of Shinsouban Mugen Gaiden clarified the different versions of the story and iterations of the protagonist, Mamiya Mugen. This series was designated "Shounen tantei B version."

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