Dossier 113 (Aka The Blackmailers)

Introducing M. Lecoq – widely believed to be the inspiration for Sherlock Holmes
Monsieur Lecoq of the French Sûreté is called to investigate a bank robbery in one of the world's first detective novels, widely credited as the inspiration for Sherlock Holmes.
The book begins with a bank robbery with Fauvel's bank losing 350000 francs. Only two people in the bank had a key and the combination to the safe and they were Fauvel himself and his chief cashier, Prosper Bertomy. The case looks black against the cashier – the only one to know about the money being put into the safe from the strong room.
As things look blacker and blacker for prosper amidst double-dealing, blackmail, and murder Monsieur Lecoq of the French Sûreté steps in to prove the young man's innocence in the face of incriminating evidence and unravel the truth behind a seemingly impossible crime.