The banking-house of Andre Fauvel has been robbed of 350,000 francs. Only two men had the key and the secret word that would open the safe where it was kept. One is the owner himself and the other is his trusted head cashier Prosper Bertomy. Both protest their innocence and the police choose to believe the respectable Monsieur Fauvel, so the suspicion is on Bertomy. Fortunately for him, the great detective Monsieur Lecoq believes him to be innocent and is ready to go to great lengths to prove his theory.

Émile Gaboriau (1832-1873) was a French writer, novelist, journalist, and a pioneer of detective fiction. He became a secretary to Paul Féval, and after publishing some novels and miscellaneous writings, found his real gift in detective novels. Gaboriau's most famous character was Monsieur Lecoq, a young police officer who was the hero in several of his novels. He also wrote a long series of novels dealing with the annals of the police court which proved very popular.