I believe the only new character who is introduced in this Chapter is Lestrade, a friend of Holmes and fellow detective.
Holmes knows that Stapleton is behind the murders Sir Charles and Selden, but he has no proof. Accordingly, he conceives a plan by which he will be able to draw Stapleton out again and get the evidence that he needs to put him away. He gains the cooperation of Sir Henry, who agrees to follow his directions explicitly, without asking why. As Holmes talks with Sir Henry, his attention is caught by portraits on the wall of the Baskerville Estate. The portrait of the infamous Baskerville ancestor Hugo bears a startling resemblance to Stapleton; it is clear that Stapleton is a Baskerville, "with designs upon the succession".
Sir Henry is invited to the Stapletons for supper the next evening, and Holmes instructs him to go alone, and plan to walk home in the dark across the moor. Holmes and Watson make arrangements so that it will appear that they have returned to London, but in reality, they will stay in the vicinity to rescue Sir Henry when Stapleton tries to do away with him as the great detective predicts. While they are waiting for evening, they call upon Laura Lyons, the woman who wrote the letter which lured Sir Charles to his death. Upon discovering that Stapleton is married, Laura reveals that he had falsely promised to marry her, and that she had written the letter according to his explicit instructions.
The proof that Holmes needs to incriminate Stapleton is almost within his grasp. Before he and Watson go to the moor to save Sir Henry from the wiles of Stapleton, they stop at the station to pick up Lestrade, who will help with the plot (Chapter 13).