The great denouement of this novel comes in Chapter 10 in Book the Third, when the letter written by Dr. Manette many years ago before he lost his mind and completely forgot how he came to be imprisoned for so long is read out as part of the case against Charles Darnay (otherwise known as Charles Evremonde). It is important to note the irony of the situation. Dr. Manette has been campaigning tirelessly for his son-in-law's release, only to find that it is his voice that is used above all to condemn him as he, his family and we as readers have the truth revealed to us about why he came to be in the Bastille for 18 years.
He narrates how he was called to the Evremonde house to tend to a young serving woman who has been forcibly raped and abused, but who is beyond healing. Then he is taken to the victim's brother who is likewise dying of a sword wound gained in a duel because of the wrong the Everemonde's committed against his sister. Because of Dr. Manette's refusal of payment (a bribe to ensure his silence) the Evremonde's imprison him. After this information is shared, Madame Defarge shares that she was the younger sister of this tragic family, which explains her hunger for revenge.