Although different in character, Sidney Carton is so physically similar to Charles Darnay, that he even impersonates him in court to invalidate the testimony of a corrupt witness. An ingenious move that will ultimately convince the jury of Darnay's innocence and save the latter's life. It's almost as if Sidney Carton and Charles Darnay were the same man. A bipolar antihero, who swings from the depth of depression to the most euphoric feats of heroism and back.
But one half will have to die so that the other half may live.
And in these lines, Sidney Carton, the morose half, the sombre dinner guest of few but intelligent words, confesses his devotion to Miss Manette through his promise of self-sacrifice. A promise he will keep to save Darnay. One final time. For Lucy. For Love. Forever.