In the Hound of the Baskervilles, Sherlock Holmes, still involved in a case in London, deems it unsafe for Henry Baskerville to go to claim his ancestral mansion alone. This is why Holmes sends Watson along with him. Holmes also wants Watson to be a spy for him. Watson's instructions are to ovbserve everybody he can and take note of anything strange or suspicious.
While at the Baskerville mansion, near Grimpen Mire--a quicksand swamp--Watson learns that the butler and his wife, Mr. and Mrs. Barrymore, have plans to depart from service for the Baskerville's because they can't bear the loss of their master, who was found having suffered a mysterious death. Then Watson hears a woman crying in anguish at night.
In the morning he sees that Mrs. Barrymore has been crying, but her butler husband denies it. All the while, Watson is writing regular reports and sending them back to Holmes in London.
Next, Watson meets Miss Beryl Stapleton who mistakes him for Henry Baskerville and makes a scene shouting and telling to go back to London because of danger. Once she realizes Watson isn't Baskerville, she won't say another word, and he can get no explanation of her outburst about danger. Finally, Watson discovers that Holmes has come to the country unbeknownst to him and has been living in disguise in a hut near the quicksand swamp, Grimpen Mire.