The mood remains mysterious, intense, and sinister throughout the novel. Much of the plot reaches life-or-death situations, which heavily influences the mood. While Collins avoids melodrama for most of the story, the characters nonetheless all feel and express their emotions very strongly.
For example, Walter is a measured and perceptive character, but he is nonetheless devastated by his impossible love for Laura, their necessary separation, her supposed death, Anne Catherick's fate, and his dwindling faith that he will manage to keep Marian and Laura safe. In addition, he can't help but admit that he is propelled forward by the motive of revenge for much of the Third Epoch. Another example is Marian, who consistently attempts to set her pragmatic self apart from women and their sensitivity but is nonetheless deeply altered by the trauma of everything she and Laura experience in the Second Epoch. Aside from Mr. Fairlie (the novel's only consistently melodramatic character) and Count Fosco (whose self-aware flamboyance entertains those within and outside of the narrative), most of the characters make readers feel uneasy when they express their emotions, intentions, and desires.
The setting also contributes to the intense mood. Collins often describes London, the novel's initial setting, as a city that is full of shadowy hiding places. Limmeridge is described in more unambiguously positive terms, but this safe haven is replaced by the dark and threatening Blackwater after the First Epoch. Walter often describes the villages he visits to inquire about the pasts of Sir Percival and the Cathericks with similar language, developing their dark and menacing atmosphere as he hunts for clues and secrets. In several scenes, characters find themselves in positions of vulnerability in relation to their settings: they are frequently out alone in the darkness of the night, walking along unfamiliar paths, or hiding in graveyards or places where their discovery would put them in danger.
The tension of the novel's characters, plot points, and setting contributes to the growing mystery and suspense. As the narrative proceeds, Collins's writing style and the sensational genre stir up an ever-growing inkling that a big reveal and a fierce confrontation are imminent. This intensifies the mysterious and intense mood.
