The manuscript Dr Mortimer has with him was given him by the recently-deceased Sir Charles Baskerville. It is a manuscript dating from the eighteenth century and relates the legend of the Baskerville family, who are seemingly haunted by a gigantic hell-hound.
Mortimer is interested in the story, and insists on reading it aloud to Holmes and Watson. It tells in lurid detail of how the hauntings began - due to the indiscretions of the fearsome Baskerville ancestor Sir Hugo, who kidnapped a peasant girl and set his dogs on her track when she tried to escape. This incurred the wrath of the supernatural hound which struck him down for his sins.
The language of the manuscript is melodramatic in the extreme, and the hound is described in sensational terms, as is clear from the following quote.
Standing over Hugo, and plucking at his throat, there stood a foul thing, a great black beast, shaped like a hound, yet large than any hound that ever mortal eye rested upon. (chapter 2)
Holmes is unimpressed by the tale, but it seems the legend has revived in modern times; Sir Charles Baskerville was apparently frightened to death by a huge hound, and a similar threat now appears to overhang his nephew, Sir Henry. It is up to Holmes, of course, to neutralise the danger and to banish the demon that seemingly stalks the Baskervilles.