Sir Henry Baskerville listened to Dr Mortimer's story with great attention and with exclamations of surprise. \"Of course I've heard of the hound,\" he said. \"When I was quite a small child, my parents often spoke of this legend, but nobody thought of it seriously. However the death of my uncle and the letter which I received this morning are very strange.\" \"How do you think to act?\" asked Holmes. \"Will you go to Baskerville Hall?\" \"Why not?\" \"The moor is a dangerous place,\" said Dr Mortimer. \"I have firmly decided to go to the home of my own family,\" answered Baskerville. \"No devil in hell or man upon earth can prevent me from going there. But it's half past eleven, Mr Holmes, and I'm going back to my hotel. If you and your friend Dr Watson can come and have lunch with me at two o'clock we'll speak more about this matter.\" \"We'll certainly come,\" said Holmes shaking hands with his two visitors. As soon as they were in the street he jumped up and cried: \"Your hat, Watson, quick! There's not a moment to lose. We must follow them.\" We ran down the stairs and into the street. Dr Mortimer and Sir Henry were walking in the direction of Oxford Street about two hundred yards ahead of us. \"Shall I run and stop them?\" I asked. \"Oh no, my dear Watson,\" answered Holmes, \"this will spoil all my plans. They mustn't know that we are following them.\" At the corner of Regent Street Dr Mortimer and his friend stopped before the window of a shop and at the same moment Holmes gave a little cry of satisfaction. \"Look, Watson,' he whispered, pointing to a cab that had also stopped on the other side of the street. There was a man in it with a thick black beard and a pair of piercing eyes. He was watching Sir Henry. Suddenly his eyes turned towards us. In a moment he had closed the window of the cab, and it moved forward quickly. Holmes looked round for another, but there was not a cab in sight. So the man with the black beard was soon far away. Dr Mortimer and Sir Henry continued their way. They didn't know that a stranger had been following them. \"Who was that man?\" I asked. \"I don't know, but I'm sure he knew that Sir Henry was at the Northumerland Hotel. This means that he has followed our young friend from the moment he came to London. So I decided to follow Sir Henry myself, hoping to catch the spy.\" \"But why did he take a cab?\" \"Because he was prepared to follow our friends in case they took a cab.\" \"What a pity we don't know the number of the cab,\" I said. \"My dear Watson, it's 2704. Now come along. The spy has gone and will not return. Let us visit one of the picture galleries to fill in the time before lunch. It's 12 o'clock now and Sir Henry doesn't expect us till 2.\"