The story seems to be a simple puzzle. 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 Andre Fauvel and the other is his trusted head cashier Prosper Bertomy. Both protest their innocence and, naturally, the police would never dream of suspecting the respectable Monsieur Fauvel, so the culprit must be the cashier. Fortunately for Prosper, the great detective Monsieur Lecoq believes him to be innocent and is ready to go to great lengths to prove his theory. Lecoq dons disguises and marshals all the officers he can to discover how the money could have disappeared if neither the banker nor the cashier is guilty.
In the end, he traces the plot to the past--a story of young lovers, separated by fate, whose actions will have repercussions in the present. And he discovers the guilt of two evil, grasping men who use their knowledge of those actions to try and ruin Prosper and his young love, Madeleine.
As I mention at the beginning of this review, the present story of the bank robbery and the actual descriptions of the Lecoq's detective work are terrific. I thoroughly enjoyed Gaboriau's narrative in those sections and my rating for this story rests completely on those portions. Where it bogs down is in the middle section where Gaboriau describes the past love affair and how it has built up to the present situation. Quite simply, that goes on WAY too long. The length completely distracts from the events we're supposed to be concentrating on--the robbery and who is framing Prosper, even though it explains the whole set up. It goes on for so long that I began to feel like I had wandered into a completely different story--one of star-crossed lovers and unhappy endings. I would have upped my rating if Gaboriau had found a more succinct way to relay that information. As it is--three and 3/4 stars for a very good mystery with romance interruptions.