When the French Revolution began, being a part of the nobility was often a death sentence. In A Tale of Two Cities, the revolutionaries are out for vengeance, as can be seen in Madame Defarge's single-minded approach to violence against the nobility. As far as the revolutionaries are concerned, Charles Darnay is a condemned man simply because he is a part of the Evrémonde family, which has so terribly abused the peasants. It does not matter that Darnay has dissociated himself from his family. The crimes of his father and uncle are enough to mark him as an enemy of the revolution.
It is unclear what would happen to Lucie Manette, Darnay's wife. There is little doubt that she would find herself in some danger, though, as the wife of this former French nobleman.
When Darnay does return to France, he is arrested as a suspected émigré and condemned to death for the sins of his uncle. His father-in-law, Doctor Alexandre Manette, attempts to defend Darnay at the trial, but it is ultimately the old man's diary describing the evils of the Evrémondes that leads to the guilty verdict.
What happens to Darnay when he returns to France gets to the heart of one of the major themes of this book. Dickens was alarmed by how many innocent people were killed merely because of their association with others during the French Revolution. Darnay and the Manettes are good people who do not harm others. However, they are caught up by the revolutionary fervor that sees guilt not in one's actions but in one's connection to one's social class. The French revolutionaries want justice, but what justice is there in condemning such good people for the injustices of their families?