[s2e2] The Hounds Of Baskerville «Direct ◉»

Determined to find a rational explanation, Sherlock realizes the "hound" isn't a physical creature, but a . He suspects a chemical agent and tests John by trapping him in a Baskerville lab, triggering a terrifying "encounter" with the beast through the base's sirens and lights. The Revelation of the Fog

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

As the fog rolls in, Frankland appears. He has leaked the gas again to finish Henry off. In the chaos, a common stray dog (enhanced by the gas to look like a monster) is shot by John and Lestrade. Frankland flees but wanders into a Baskerville minefield, meeting a fiery end. The Aftermath Determined to find a rational explanation, Sherlock realizes