When Monsieur Leblanc—Jean Valjean—goes to the Gorbeau building and walks right into a trap, Jondrette reveals that he is the inn-keeper Thenardier. Marius, who is spying from the next room, uncovers the ironic secret about the man who saved his father's life at the Battle of Waterloo:
This man was that Thénardier, that innkeeper of Montfermeil whom he had so long and so vainly sought! He had found him at last, and how? His father’s savior was a ruffian! That man, to whose service Marius was burning to devote himself, was a monster! That liberator of Colonel Pontmercy was on the point of committing a crime whose scope Marius did not, as yet, clearly comprehend, but that resembled an assassination! And against whom, great God! What a fatality! What a bitter mockery of fate!
This is a moment in which a long-running instance of dramatic irony is finally fulfilled—readers have waited hundreds of pages for this to happen. For a long time, Marius has felt a certain duty and devotion to this unknown Thenardier who saved his father’s life. When he learns that Thenardier is nothing more than a common criminal, his world is shattered. This is also an instance of situational irony, since the man who was a hero for the ages only moments ago in Marius’s mind has now been revealed to be a lowly criminal. The irony that his father's hero turned out to be a dirty thief is painful for Marius, who is so strongly ruled by ideals and dignity. Marius is very shaken now that the hard truth has been revealed.