Deep and Dark and Dangerous is centered on a decades-long secret kept by sisters Claire and Dulcie. To keep their involvement in Teresa’s death hidden, Dulcie swore her younger sister to secrecy, but their silence has come with a cost—not only for them and their families, but also for the residents of the Sycamore Lake area. Hahn makes the enumeration of these costs one of the novel’s central themes.

When Ali discovers the photograph and starts asking questions, Claire lashes out at her because her inquisitiveness threatens to puncture the tenuous shroud Claire has kept over her past with Teresa. For Claire, especially, the toll of keeping this secret has been enormous, resulting in issues with her health. Although a sensitive child even before the incident, Claire becomes even more so afterwards, experiencing physical and mental health issues like severe headaches, anxiety, stress, and depression—the latter of which required clinical treatment. In Claire’s example, Hahn represents both the psychological and physiological ways that the toll of keeping such secrets can manifest.

The sisters’ secret also threatens the safety of the next generation of Thorntons to visit the lake. The irony of not knowing who Teresa was or what she looked like is that Ali and Emma are unable to recognize her ghost in the form of Sissy.