It is about an amateur detective unmasking a group of German spies in France on the eve of World War I.

The same evening Gerfaut, after having bombed the factory sends an anonymous note to Richard asking him to meet him at a chapel in ruins.

Before the train journey takes place, Aubry is warned by Chantecoq that Richard is dead and that he has met an imposter.

Emma disguised as a widow takes place in the same compartment as Gerfaut and Chantecoq and together the two spies overpower him.

They are very happy to discover that Aubry is in fact their worst enemy Chantecoq and they ship him to Germany in a trunk.

Chantecoq is invited to have lunch with the Crown Prince on the terrasse of one his castles overlooking a lake separating Germany from Switzerland.

He first exults over his easy escape but is rapidly disillusioned when the Prince's men start shooting at him and chase him with a speed boat.

In an epilogue, Chantecoq is sitting in a garden with his friends and a flashback shows how he managed to arrive in Switzerland.

The location of the action is Bar le Duc was highly symbolic given that Verdun was nearly surrounded by the Germans and the only road connecting it to the rest of France was the road to Bar le Duc, which was the place where supplies and men were concentrated before being sent to the front.