At the beginning of the chapter A Hand at Cards, Miss Pross and Jerry Cruncher walk into a bar...(insert bad punchline here). Actually, they meet two people whom they have not seen in a long time--a spy named John Barsad (whom we have seen before as a witness against Charles Darnay in the earlier part of the book) who also happens to be Miss Pross' prodigal brother, Solomon.
They also meet Sydney Carton, who has made his way to Paris for unknown reasons. Sydney Carton, having a keen memory for faces knows exactly who John Barsad is. Illustration by Rene ben Sussan from the Heritage Edition.
Sydney Carton blackmails John Barsad, whom he has seen at court in England and known as a spy. Carton has also seen Barsad acting as a turnkey at the prison La Force--his history as a spy for the English Aristocracy will doom him if the French Revolutionaries ever discover it. Jerry Cruncher, meanwhile has just made a startling connection, causing his hair to stand on end in a disturbing manner. This illustrator is uncredited in the editions in which I have seen his illustrations. This set of illustrations is pretty common and I've seen it a lot in Brittish editions from the early 20th and late 19th century.