Dr. James Mortimer ask Sherlock Holmes to investigate the death of his friend, Sir Charles Baskerville. Sir Charles died at his Devonshire estate, Baskerville Hall, and Mortimer now fears for Sir Charles's nephew and sole heir, Sir Henry Baskerville. 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.