Dr James Mortimer asks Sherlock Holmes to investigate the death of his friend,
Sir Charles Baskerville. Sir Charles was found dead on the grounds of his
Devonshire estate, Baskerville Hall, and Mortimer now fears for Sir Charles's
nephew and sole heir, Sir Henry Baskerville, who is the new master of
Baskerville Hall. The death was attributed to a heart attack, but Mortimer is
suspicious, because Sir Charles died with an expression of horror on his face,
and Mortimer noticed "the footprints of a gigantic hound" nearby.