In Chapter X of Book the Third of A Tale of Two Cities, Charles Dickens employs the literary technique of Back-story with the letter of Dr. Manette, formerly prisoner One Hundred and Five North Tower of the Bastille, that is discovered by Ernest Defarge upon the revolutionaries storming of the old political prison. This letter of Manette's provides background information that is most significant both to character and to events of the plot.
The letter is one that Dr. Manette has written in his tenth year of incarceration, using the rusty iron point of a nail that he scrapes into soot and charcoal. Manette explains the history of his being imprisoned after having been taken by the Evremonde twin brothers who were armed. In a type of barn, Manette finds both a dying young woman and a boy of seventeen. One of the Evremonde twins have slain him and caused the woman her death. After they both die, the brothers offer Manette money, but he refuses it. On the next day after his return home, Dr. Manette finds a bag of gold outside his door; nevertheless, he pursues his intention to write to privately to the Minister of the past affair. Before he sends his letter, however, Manette has a visitor, the wife of one of the brothers Evremonde. She worries that God will punish her son for the sins of his father; in her hopes of averting this punishment she seeks the living sister. But, Manette cannot help her. So, he sends his letter. On that night, he is tricked into thinking he is called to aid someone ill, but, instead, is captured and taken to prison, the prison from which he records his history of the fateful incidents.
Later in the novel, the testimony of this letter is crucial to incidents involving Madame DeFarge and Dr. Manette and Charles Darnay. Dr. Manette's letter as back-story is also central to clearing up the identities and occurrences involving Madame Defarge and Darnay.