A Tale of Two Cities is at once one of the most popular and most atypical of Dickens' novels. For one thing, it's an historical novel, a form Dickens employed only one other time in Barnaby Rudge. It's also considerably less funny than his other works; humor is still present, but much darker and much less frequent. Its popularity doubtless arises from its accessibility (it is far shorter and less dense than other Dickens novels) and from the beauty of the story, particularly as it concerns the incomparable and enigmatic Sydney Carton.
The two cities of the title are Paris and London at the end of the 18th century during the French Revolution, between which the whole action is divided. Sydney Carton, a dissolute but brilliant lawyer, represents London, while the noble but disinherited aristocrat Charles Darnay represents Paris. The initial tension between the two represents not only strained British/French relations, but the class struggle of the Revolution. The two form an unlikely friendship despite Carton's early jealousy for the more successful man and their love for the same woman, Lucie Manette. Typical Dickensian plot intricacy develops as Darnay learns of a secret link between himself and Lucie's father, several characters' illegitimacy is revealed, and Darnay and Carton are drawn into the chaos of the Revolution.
There's plenty of adventure here, as well as romance and mystery and social commentary. Dickens' pet theme of class-based injustice is prominent as the plight of the French peasantry is described in horrific detail. His poetic language is at its best, with one of the most famous opening sentences in all of literature ("It was the best of times, it was the worst of times..."), as well as one of the greatest closing lines. Carton's action on behalf of Darnay at the novel's conclusion is one of the most classic moments in literature. But it isn't any of these elements (common throughout the Dickensian canon) that distinguish A Tale of Two Cities.
The real greatness of the novel lies in its exploration of personal redemption and selfless love, particularly in the character of Sydney Carton. At the beginning he is a true existentialist, living in despair and resignation despite his talents and brilliance. And while one might be tempted to suppose it's his love of Lucie that leads him out of his personal Slough of Despond to a moral resurrection, Dickens clearly indicates it is actually his brotherly admiration and love for Darnay that moves him out of himself toward Christ-like goodness. This is an important point—if it were simply his love for Lucie that led him to the ultimate self-sacrifice we could dismiss it as merely romantic and thus ultimately, at least in some sense, selfish. The fact that Carton is motivated by love of Darnay and a sense of moral certainty lends not only meaning but genuine moral purity and selflessness to his sacrifice, as one who stands to gain nothing from his own loss.
One of the most modern of Dickens' stories, A Tale of Two Cities is in many ways (and perhaps because of its modernity) also one of his darkest. It is violent in a way few of his works are, and while it ends beautifully it also ends tragically. None of the neat denouement for Carton or Darnay, nothing in fact past bloody death and sadness. Of course Darnay has the girl and the prospect of a happy life in England, but even this is a half-happiness—the destructive Revolution is still spreading terror at the end, and Darnay may be alive and happily married but he must finish his days in exile from the country of his birth. Dickens would explore this realism more fully in Great Expectations and Our Mutual Friend, but here it is a marked departure from his formerly happy (if often bittersweet) endings.
Great in its own right, A Tale of Two Cities is one of the preemeninent works among Dickens' vast authorship. A novel of great pain and beauty, it forces us away from any fairy tale nostalgia we may have for "romantic" historical periods, and examines instead the consistency of human nature with its capacity to endure and inflict suffering, while celebrating the insistence of some people that good can exist and their efforts to see that it does, even at ultimate cost to themselves.