A tale of two cities

This is only a short answer space. After Lucie marries Charles Darnay, the novel tends to be concerned with their struggle to keep their family together. When Darnay laments his own death sentence, it is for the sake of his family, not for his own sake. The final triumph is the sacrifice of Carton, a man who is unattached to any sort of family, who thus preserves the group consisting of the Doctor, Lucie, her husband, and her children.