Five years after arriving in London where his daughter hopes to restore his sanity and health, Dr. Manette and Lucy and Mr. Lorry are called as witnesses in a trial at Old Bailey. Their former acquaintance on the coach from Dover, Charles Darnay, has been teaching French while living in England, and is accused of treason by reason of his trips between the two countries. In Chapter 2 of Book I, Dickens describes the blood-lust amusement of the spectators at Old Baily where Lucie Manette falteringly gives damaging circumstantial evidence against him. However, the prosecution's case fails when a witness cannot positively identify Darnay because of his resemblance to Sydney Carton, a lawyer in the courtroom. So, Darnay is acquitted and the crowd
came pouring out with a vehemence that nearly took him [Jerry] off his legs, and a loud buzz swept into the street as if the the baffled blue-flies were dispersing in search of other carrion.
This passage is significant because it is a prelude to the crowd frenzy in the other city, Paris, and the blood lust of the French peasants. Dickens's metaphor of the "blue-flies" appears earlier in the passage about the trial:
When the Attorney-General ceased, a buzz arouse in the court as if a cloud of great blue-flies were swarming about the prisoner, in anticipation of what he was soon to become....
Then, when the verdict is not hanging--and the "accused,...was being mentally hanged, beheaded, and quartered, by everybody there..."--the flies seek other victims as the "blue flies" of hatred swarm through France.
In addition, with the similarity of Darnay and Carton this passage also introduces the motif of doubling that Dickens uses in this novel.