Two men, two cities. It's the classic tale of rivals in love bound by brotherly friendship, set against the chaotic backdrop of the French Revolution as seen through both English and French eyes. Many readers are doubtless drawn to A Tale of Two Citites because it's short, but they read the whole thing because it's beautiful and compelling.
Dickens only wrote two historical novels (this one and Barnaby Rudge), but he mastered the genre. The two dangers of historical fiction are to focus too much on historical detail, events and characters, or to simply write a contemporary story but set it in a past era. Dickens falls prey to neither, and the result is a story of pathos, atmosphere, and excitement.
Sydney Carton is a dissolute but brilliant lawyer, while Charles Darnay is a noble but disinherited aristocrat. The initial tension between them represents both strained British/French relations and the class struggle of the Revolution. The two become unlikely friends despite Carton's early jealousy for Darnay and their mutual love of Lucie Manette. Dickensian intricacy ensues as secret links emerge, true identities are revealed, and Darnay and Carton are drawn into the chaos of the Revolution.
There's plenty of adventure, romance, and mystery. Dickens's pet theme of social injustice is prominent as the plight of the French peasantry is described in horrific detail. His poetic language is at its best, with the famous opening sentence ("It was the best of times, it was the worst of times...") and a near-perfect last line. But none of these distinguish A Tale of Two Cities.
The greatness of the book is its exploration of selfless love. Syndey Carton begins a true existentialist, in despair despite his many talents. But it isn't his love of Lucie that leads him out of his personal Slough of Despond to a moral resurrection: it's his brotherly love for Darnay that moves him toward Christ-like goodness. That Carton is motivated primarily by love of Darnay (and not Lucie) lends meaning and selflessness to his sacrifice, as one who stands to gain nothing from his own loss.
This is a dark tale. Its violence is atypical of Dickens, and while it ends beautifully it also ends tragically. No neat denouement—just death and sadness. 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.
A Tale of Two Cities 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.