Pondicherry Lodge looks imposing in the moonlight; it is protected by a high stone wall topped with broken glass, with only a heavy iron door for entry. The doorman, McMurdo, is reluctant to let in anyone but Thaddeus. Holmes realizes that he knows McMurdo, whom he has previously fought against in an amateur boxing match. McMurdo now happily lets them inside. The doorman explains that Bartholomew has not left his room all day.
Pondicherry Lodge is palatial and castle-like, aligning with the conventions of the gothic genre to build suspense. The grumpy doorkeeper fits with these conventions, too. Holmes often has a way of turning a situation quickly around in his favor, often slightly implausibly, as is the case here.
They walk up a dimly lit path to the house. Thaddeus goes in meets the house keeper, Mrs. Bernstone, who has been crying. Miss Morstan and Watson instinctively hold hands in the dark as they look at the grounds with Holmes. The earth has been dug up all over the place in search of the treasure.
Thaddeus comes rushing out, shouting that something is wrong with Bartholomew. Mrs. Bernstone expresses relief at seeing them, explaining that she saw a terrifying sight when she peeped through Bartholomew’s keyhole to check if he was okay.
This suggestion of a “terrifying sight” increases the tension, setting up a horror story-esque reveal. The locked room is also a classic convention of the gothic genre.
They go upstairs. Holmes tries to open the door to Bartholomew’s room, but it is locked. He and Watson look through the keyhole, surprised to see Bartholomew’s face staring straight at them, fixed in a rictus grin. They break the door open and go inside.
Bartholomew’s room looks like a “chemical laboratory.” They notice a set of steps leading up to a hole in the ceiling, and a long coil of rope at the foot of the steps. Bartholomew is stiff and cold; he’s been dead for hours. Holmes picks up a note by the body, which once again reads “the sign of the four.” He also notices a “long dark thorn stuck” in the side of Bartholomew’s head.
Doyle doesn’t explain why the room is set up as a laboratory, but it would have likely reminded readers of Frankenstein’s laboratory in Mary Shelley’s gothic story of the same name. The long dark thorn, one of Tonga’s blow darts, suggests an evil, foreign influence—literally, with the “foreign” poison entering the “homeland” of Bartholomew’s body.
Surveying the scene, Watson is baffled, but Holmes says he only needs “a few missing links to have an entirely connected case.” Thaddeus remarks anxiously that the treasure has gone. Holmes instructs him to go the police and report what’s happened.
Watson represents the reader, who is unlikely to have garnered much of an understand of the case, and contrasts with Holmes’ intellectual ability.