Sarcasm is employed in the narration of Chapter I with the discussion of the attempted escape from France by the aristocrats. [Examples are italicized.]
The author, Baronness Emmuska Orczy, an aristocrat herself, describes with sarcasm the macabre enjoyment of the people who watch as the aristocrats, who have been labeled as "traitors to the people" after the Revolution, attempt to flee the country and escape the guillotine. This treatment of the sadistic enjoyment of the peasants is certainly ironic (sarcastic) and intended by Orczy to mock the formerly oppressed peasants, who now themselves become the oppressors.
The "fun" begins when Sergeant Bibot, who has a "wonderful nose for scenting an aristo in the most perfect disguise," does not immediately arrest the aristocrats, but instead toys with them as a cat often plays with a mouse before killing and eating it. This "keen sense of humor" amuses the crowd who stand near the barricades and watch as Bibot often lets his "prey" past the gates in order to make them think they have made their escape. Soon, however, the aristocrat is marched back. To the people this is "extremely funny" because, often as not, the prisoner is a woman, "some proud marchioness, who looked terribly comical" as she has been stripped of her disguise and realizes that she will receive but a summary trial and after this, she will feel "the fond embrace of Madame la Guillotine."
Every day Bibot has the distinct "satisfaction" of apprehending royalists and returning them to the Committee of Public Safety that is presided over by "that good patriot" Citoyen [Citizen] Foucquier-Tinville. But, there is one man who somehow slips past the watchful eyes of the guards. He is an Englishman, the "accursed Scarlet Pimpernel" who cleverly escapes by disguising himself in the most unsuspected personas. Boasting that he would never be so foolish as to let this Englishman go past him, Bibot tells the crowd of one daring escape which cost a guard the punishment of the guillotine. It seems there was a daring escape in which the Scarlet Pimpernel was dressed as a guard and the aristocrats as soldiers while a harmless cart passed out of the city. Hearing this, the crowd becomes silent.
The story savored of the supernatural, and though the Republic had abolished God, it had not quite succeeded in killing the fear of the supernatural in the hearts of the people.
But, further sarcasm comes at the expense of Bibot, who has previously ridiculed the guard deceived by the Scarlet Pimpernel. For, it seems that Bibot has let pass a cart driven by "an old hag" who has told him that her son has had the plague so that he would not wish to be near her or the cart. Ironically, this cart has been allowed by Bibot to pass through with aristocrats hidden in it. And, the "old hag" has been none other than the "accursed Englishman" himself.