What is the theme of the playscript Valley of Fear by Arthur Conan Doyle. Please explain in a sentence.

The Valley of Fear begins with Sherlock Holmes, the famous detective, and Dr. Watson, his comrade and roommate at 221B Baker Street, in mid-conversation. Holmes is puzzling over an encrypted message that he received from Porlock, an associate of Holmes’ nemesis, Professor Moriarty. A second 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.