Fearing that the recent death of his friend, Sir Charles Baskerville, is linked to a family legend of a supernatural hound from hell, Dr James Mortimer calls on Sherlock Holmes (Tom Baker) and the ever-willing Dr Watson (Terence Rigby) in a bid to prevent the same fate befalling the new heir to the estate, Sir Henry Baskerville (Nicholas Woodeson). A mysterious note and the baffling theft and return of some boots set Holmes and Watson Dartmoor-bound, but death awaits on the remote and treacherous fog-bound moors as an eerily-glowing menace prepares to strike...