The story closes with a chilling realization. Sherlock’s "Mind Palace" vision of in a prison cell was actually a lingering effect of the gas—a shadow of his greatest fear. As Henry finally finds peace, the episode ends with a teaser: Mycroft releasing Jim Moriarty from a holding cell, where the villain has written "SHERLOCK" all over the walls. O.U.N.D. project?
They rush back to the Hollow to find Henry on the verge of suicide, driven mad by the gas. Sherlock explains the truth: Henry didn't see a monster twenty years ago; he saw in a gas mask and a "H.O.U.N.D." t-shirt, murdering Henry's father over the secret of the gas. Henry’s child-mind interpreted the mask and the red goggles as a demon dog. [S2E2] The Hounds of Baskerville
Back at the inn, the unthinkable happens. Sherlock Holmes—the man of pure logic—is trembling. He confesses to John that he saw the hound. For the first time, Sherlock cannot trust his own senses. This doubt leads to a breakdown of his "Mind Palace," nearly destroying his friendship with John as he lashes out in fear. The story closes with a chilling realization
That night, Sherlock and Henry venture into Dewer’s Hollow. In the darkness, amidst the swirling mist, they see it: a massive, snarling beast with glowing eyes. The Great Detective’s Doubt Sherlock explains the truth: Henry didn't see a
Sherlock, initially dismissive, is hooked when Henry mentions "hound"—a word that triggers a strange flicker in Sherlock's own mind. The Beast of Baskerville
The investigation centers on , a top-secret military research base nearby. Rumors of genetic mutations and "super-dogs" swirl among the locals. Using Mycroft’s high-level security clearance (stolen, of course), Sherlock and John infiltrate the facility. They encounter Dr. Stapleton , a geneticist with a missing glowing rabbit, and Dr. Frankland , a friendly scientist who was a close friend of Henry’s father.