Sherlock Holmes and Dr. Watson receive a letter from an informant known by the pseudonym Fred Porlock. Porlock is part of Professor Moriarty's criminal organisation. The letter is written in a numeric code, and Holmes realises that the numbers refer to words in a book, by page and column. They decode the letter which warns that John Douglas of Birlstone House is about to be murdered. After they have deciphered the message, Inspector MacDonald of Scotland Yard comes to consult Holmes. MacDonald is astonished when he sees the message, because it has pre-empted his news: a man called John Douglas has indeed been mysteriously killed in Sussex. MacDonald demands to know the true identity of the informant who predicted the crime, but Holmes does not know it, because he promised not to try and find out who 'Porlock' really is. Holmes can only tell MacDonald that the informant works for Professor Moriarty...