The evil mastermind known as the Fiend with Twenty Faces is fed up with Kogoro Akechi and those meddling kids from the Boy Detectives Club. Determined to exact his revenge, the Fiend embarks on a crime spree, stealing top secret documents and a priceless work of art, all the while kidnapping and tormenting anyone who stands in his way.

The ingenuity of this archvillain knows no bounds. Living up to his nickname, the Fiend dons one disguise after the other. He soon has the police chasing their tails, and even shows up to investigate his own crime! Obsessed with his vendetta, he pursues his quarry through haunted houses and limestone caverns inhabited by giant bats.

The Fiend won't be satisfied until he finally confronts Detective Akechi and the members of the Boy Detectives Club in a life-or-death struggle deep underground in the dark.