Who come to solicit help from holmes and watson

A message from Porlock, which was supposed to contain the key to the coded message, soon arrives, stating that he (Porlock) is afraid of Moriarty’s wrath and will not reveal the purpose of his correspondence. Holmes soon deciphers the message regardless, and discovers that it warns that Douglas of Birlstone is in imminent danger. The two are soon visited by MacDonald, a policeman friend of Holmes, who informs them that this same Douglas has been murdered.