In the prison, 52 people, including Charles, await execution that day. Charles writes a final letter to Lucie, in which he says that he did not know about her father's history and that he believes Dr. Manette was unaware of his damning letter. Charles writes much the same to Dr. Manette. He also writes to Mr. Lorry, but never thinks to write to Carton.
Neither Charles nor Dr. Manette were aware of their real legacies. They don't control their own destinies. Charles has underestimated Carton before. The fact that Carton is under no obligation to make his sacrifice only increases its symbolic power.
Suddenly John Barsad opens the cell door and lets in Carton. Carton tells Charles to start changing clothes with him. Then Carton dictates a letter for Charles to write, in which he asks "someone" to remember him and is grateful to have the chance to prove himself.
In Book 2, Chapter 4 Carton envied Charles. Now he becomes Charles by literally sacrificing his identity to save Charles's life. The "someone" in the letter is Lucie.
As Charles writes, Carton waves the packet of drugs under his nose. Charles passes out. Carton finishes swapping their clothes and Barsad carries Charles, now disguised as Sydney Carton, back to Mr. Lorry.
Charles has been helpless to stop history, and is not just passive, but actually unconscious, during his escape.
Soon the guards arrive and take Carton, whom they think is Charles Evrémonde, out to join the other condemned prisoners. A young woman, who was wrongly accused and convicted, asks him if she can hold his hand. Suddenly, the women realizes that he is not Evrémonde. "Are you dying for him?" she asks. "And his wife and child," Carton replies. Carton promises to hold the woman's hand until the end.
The young girl reveals how corrupt and merciless the republic's tribunals are. Her innocence also lets her recognize Carton for who he is: a figure of Christ, giving his life to save others. Holding Carton's hand suggests how the girl's faith will sustain her.
At the Paris barricade, guards check the papers of the passengers in a carriage: Mr. Lorry, Dr. Manette, Lucie, and "Sydney Carton," who is unconscious. They wave the carriage through.
Just as Mr. Lorry smuggled the infant Lucie out of Paris, he now transports these mostly helpless passengers to safety.