The mood of The Sign of the Four is dark and suspenseful. At its heart, this novel is a murder mystery, and the story is as complicated, violent, and even horrifying as one should expect from such a subject. Doyle takes care to adhering to the conventions of the gothic genre—a prime example of these gothic themes comes at the discovery of Bartholomew's body in Chapter 5:
I stooped to the hole and recoiled in horror. Moonlight was streaming into the room, and it was bright with a vague and shifty radiance. Looking straight at me and suspended, as it were, in the air, for all beneath was in shadow, there hung a face—the very face of our companion Thaddeus. There was the same high, shining head, the same circular bristle of read hair, the same bloodless countenance. The features were set, however, in a horrible smile, a fixed and unnatural grin, which in that still and moonlit room was more jarring to the nerves than any scowl or contortion.
The play with lighting, the gradual and terrifying discovery of Bartholomew's body, and the description of his ghastly expression create and sustain a macabre mood that is central to gothic fiction. In the case of The Sign of the Four, such a mood heightens the suspense for the reader and ensures that the central mystery of the Agra treasure is shrouded in an appropriate amount of dread and gloom.
