What mystery are Sherlock and Watson trying to solve in chapter one? What is their deducation? Thank you

Holmes asks why Dr. Mortimer has called on him, and Mortimer tells him that he has a most serious and extraordinary problem. Apparently a demon hound chronicled in an ancient manuscript haunts the Baskerville family. The latest Baskerville to have been killed under mysterious circumstances, perhaps the "demon hound", is Sir Charles. Dr. Mortimer wants Holmes and Watson to investigate.