Sherlock and John visit Baskerville, a military research base, in “The Hounds of Baskerville.”
In the episode, Baskerville is presented as a Ministry of Defence research facility on or near Dartmoor. Sherlock uses Mycroft’s security credentials to gain access, and the visit takes the investigation beyond local folklore into secret military science. The base’s laboratories become central to the mystery when John encounters what seems to be the monstrous hound.
The facility is linked to H.O.U.N.D., an abandoned CIA project intended to create a hallucinogenic anti-personnel chemical weapon. Dr. Frankland has continued related experiments in secret, using the chemical to provoke terrifying visions. This explains why different characters perceive the hound and why Sherlock initially doubts his own senses.
Baskerville is fictional, although the production based its design on real military research centres, particularly Porton Down. It should not be confused with Barts, Sherlock and John’s London hospital, or Appledore, the later-series home associated with Eurus Holmes.