Why does Dr. Manette believe his past imprisonment "all tended to a good end, ...it was not mere waste and ruin" in Book 3, Chapter 4 of A Tale of Two Cities?
Dr. Manette believes that his eighteen-year imprisonment in the Bastille "all tended to a good end" because he will be helpful in restoring his daughter Lucie's husband to her.
Dr. Manette is able to get Charles Darnay moved where he is not confined alone; also, he is able to see Charles every week so that he can bring messages to Lucie from her husband. He even feels that he will be influential in effecting Charles's ultimate release. However, the blood thirst of the revolutionaries becomes so insatiable that the prisons "gorged with people who committed no offence" (Bk. 3,Ch.4) and nothing gets accomplished toward Charles's release. Nevertheless, the doctor becomes renowned as the "Bastille Captive" and is set apart because he has suffered at the hands of the aristocrats, those villains who are now the prisoners among whom Dr. Manette walks. In fact, the good physician is almost considered a "[S]pirit moving among mortals."
After over a year, Charles Darnay is moved to a cell with a window where at a certain time of the day Lucie can stand beneath this window and wave to her husband. However, one day her father takes Lucie aside to tell her some unfortunate news: "Charles is summoned for tomorrow." (Bk. 3, Ch.5) Added to this, the ominous figure of Madame Defarge appears, then is gone "like a shadow over the white road." (Bk.3, Ch. 5)
Dr. Manette's unjust eighteen-year incarceration has made him a kind of symbol or folk hero to the revolutionaries who have just overthrown the government which imprisoned him. This status gives him a certain influence which he uses to protect his daughter's husband, Charles Darnay, who is in prison now and in danger of being executed by the new regime. Dr. Manette finds that his years in chains have given him a strength and power that he will use to restore Darnay to his beloved daughter Lucie.