Dr. Richard Mortimer (Francis de Wolff) asks Sherlock Holmes (Cushing) and Doctor Watson (Morell) to investigate the death of his friend Sir Charles Baskerville; he believes that Sir Charles was killed by a monstrous hound that, legend has it, killed his ancestor Sir Hugo Baskerville (David Oxley) centuries before. Intrigued, the pair go to meet the new owner of Baskerville Hall, Sir Henry (Lee). Holmes remembers that he is going to be away on another case, and leaves Watson to look after Sir Henry. A tarantula attacks Sir Henry briefly in London; Holmes suspects foul play. Before he leaves, Holmes reminds Watson not to let Sir Henry go out onto the nearby moor - where Hugo and Charles died - after dark.