In The Tale of Two Cities, Charles Darnay is a native of France who is now happily living in London. When he was last in France he denounced his claim to his aristocratic title and home. The Revolution had not yet begun, and he did what he felt was right to absolve himself of his home and heritage--everything but make it official, that is. Things changed after he left.
Charles must go back. What he doesn't understand is how things have changed and the thirst for aristocratic blood in France since he was last there. If he had known, he still would have gone; however, it's likely he would have gone about it differently.
He went back to France because an old family servant, who was a friend to him, sent him a letter begging for help. Although he knew the risks of going back he went anyways because this servant, Gabelle, had been imprisoned and he was asking for Darnay to come and help him be free.