From the first paragraph, Charles Dickens establishes light and dark as two of the key contrasts of the era he explores in A Tale of Two Cities.: “it was the season of Light, it was the season of Darkness.” Within the sets of paired opposites, these concepts are immediately followed by “hope” and “despair”: “it was the spring of hope, it was the winter of despair.” Throughout the novel, darkness and shadows are associated with secrets, dishonesty, illegality, immorality, and death. The shadowy underworld of grave-robbing, mercenary bankers, and the revolutionary terror are manifestations of the darkness of evil.
The “secret and mystery” that each human constitutes for others is explored in Book the First, chapter 3. The narrator describes the coach passing through “the shadows of the night” that affect the men and even the horse. These shadows are revealed in the “wandering thoughts” of the passengers—one of whom is Lorry, dreaming of rescuing a man who had been “buried alive.” The sunlight when he wakes up is associated not only with life, but with resurrection, an important theme in the whole novel.
Dr. Manette—whose imprisonment is later revealed as the meaning of this dream—is among those with a terrible secret, which "casts a shadow beyond" him. Jerry Crutcher literally conducts his clandestine body snatching under cover of dark, while metaphorically his activities are deemed morally reprehensible.
The contrast between shadow and light is applied consistently to the foil characters of Madame DeFarge and Lucie Manette. Book the Third has chapters entitled “The Shadow” and “The Substance of a Shadow.” The older woman is described as passing "like a shadow over the white road," where Lucie stands in the snow. Madame is associated with evil intent, including revenge, which is often directed against Lucie. The lovely young woman is presented as a “golden thread”—literally a reference to a strand of hair—whose goodness and purity provide the hope that sustains her father and brings courage to the two men who love her.