Sherlock Holmes: The Man with the Twisted Lip

Dr. Watson is asked by a friend of his wife to find her missing husband, who is an opium addict. Dr. Watson tracks him down to an opium den, where, to his great surprise he also finds Sherlock Holmes.
Holmes is investigating the disappearance of Hugh Boone, who he believes has murdered Mr. St. Clair. However, several days later Mr. St. Clair's wife receives a letter from her husband that forces Holmes and the police to reconsider the case.