Reluctantly, after an illness, Holmes takes a holiday in Cornwall, with Watson for company. Then one day he finds himself investigating a mysterious death... after a quiet evening at home playing cards, three local inhabitants have been found in the morning in a terrible way: the two brothers overtaken by sudden mental illness, babbling incoherently, and their sister dead, with her face twisted into a look of terror. Holmes is delighted with the chance to bury his holiday boredom by taking on the case.

On a visit to Wisteria Lodge, Scott Eccles gets a strange reception. A mysterious note arrives for his host, Garcia, and the next morning Garcia and his servants have vanished without trace. Eccles consults Holmes, who decides to treat the case as a murder investigation, but it appears that Holmes and Watson are on a wild goose chase.

The body of one Cadogan West is found in an underground tunnel near Aldgate Station, with his skull smashed, and Holmes is puzzled by the interest his brother Mycroft is showing in the case. Mycroft explains that the top secret plans for a new submarine were in the dead man's pocket, but three of the ten pages were missing when the body was found. Mycroft, on behalf of the British Government, asks his brother to find the missing pages.

Sir Charles Baskerville is found dead on Dartmoor, with his face contorted in terror and the footprints of a huge dog near his body. On arrival in England, Sir Charles's American heir, now Sir Henry Baskerville, engages Holmes to come to Devon and get to the bottom of the age-old legend of the Hound of the Baskervilles - a phantom which kills.From the outset, Holmes is convinced there is a human murderer at work and he lays a deep plan to foil him.