Plot summary
Sherlock and John are contacted by Henry Knight ( Russell Tovey ), a man traumatised by the death of his father by a monstrous hound on Dartmoor years before. Investigating Dewer's Hollow, a local spot where the beast was allegedly seen, as well as the nearby Ministry of Defence testing site Baskerville, Sherlock and John uncover a conspiracy wherein one of the Baskerville scientists, Dr. Frankland ( Clive Mantle ), is continuing the work of H.O.U.N.D., an aborted project to create a hallucinogenic gas for military use. Sherlock and John discover that the legendary hound is an ordinary dog used for publicity that the hallucinogenic gas makes appear as a demonic monster.
The "hound" that killed Henry's father was actually Frankland wearing a red-lensed gas mask and a T-shirt bearing the logo of the H.O.U.N.D. group. Confronting both the dog and Frankland at Dewer's Hollow, John and Lestrade shoot the dog. Frankland attempts to flee, but dies when he runs into a minefield. In the final scene, Mycroft releases a confined Jim Moriarty. Based on the novel The Hound of the Baskervilles.
Credits
Adapted from Wikipedia contributors, List of Sherlock episodes. HTML formatting and reference markers removed.