Early in A Tale of Two Cities, there is an indication of the intellect and the integrity of Sydney Carton as he cleverly establishes a flaw in the credibility of a witness who supposedly recognized Charles Darnay when he purportedly uttered his treasonous words on the Dover carriage. Carton raises doubts as to whether the witness truly saw Darnay, and thus saves the accused from prison. This act foreshadows Carton's final heroic act at the novel's end.
In Book the Third, after Charles Darnay returns to France in order to aid Gabelle, and he is arrested, Lucie faints, unaware even of Sydney's presence. Little Lucie entreats him,
"Oh, Carton, Carton, dear Carton! … Now that you have come, I think you will do something to help mamma, something to save papa! Oh, look at her, dear Carton! Can you, of all the people who love her bear to see her so?"
He who has promised Lucie in Chapter XIII of Book the Second,
For you, and for any dear to you, I would do anything. If my career were of that better kind that there was any opportunity or capacity of sacrifice in it, I would embrace any sacrifice for you and for those dear to you....
cannot, indeed, bear to see Lucie this way. True to his pledge of loyalty and life, Sydney Carton recognizes a turnkey at the prison where Darnay is being held, a certain John Barsad who was a witness in Darnay's trial for treason against the British crown. Identifying him as a double-spy, Darnay threatens to reveal Barsad's identity to the French bonnets-rouges (the Revolutionaries) unless Barsad assists him by allowing him into the prison so that he can take Darnay's place. Courageously, then, Sydney Carton makes the ultimate sacrifice in his love for Lucie, honoring his pledge of devotion to her.
When the prisoners are charged to walk out of the prison and step into the tumbrils that will take them to the guillotine, Darnay, who has advised Mr. Lorry to quickly get the Darnays out of Paris, holds the hand of the seamstress until he must mount the stairs to his death. Aware that he has attained honor for fulfilling his loving pledge, Carton "stands tall" as he ascends the steps where Death waits for him; in his heart, Sydney Carton hears saving words,
"I am the Resurrection and the Life, saith the Lord: he that believeth in me, though he were dead, yet shall he live...."
as he knows "It is a far, far better thing that I do, than I have ever done...." In the presence of evil and adversity, Sydney Carton has supreme courage; he keeps his pledge to Lucie Manette and becomes the sacrificial victim for her husband, whom she loves.