Sherlock Holmes (Robert Downey, Jr.) seems more than distracted at the news that his long-time companion Dr. John Watson (Jude Law) wants to marry a woman, which has sent Holmes into a funk more than any addiction to cocaine. Only a new case can awaken Holmes, and his acuity returns when he finds a new mystery. Lord Blackwood (Mark Strong), a blackguard, returns from apparent death to spread evil in Victorian England and blow up Parliament once and for all with his magical powers. The case is enough to return Watson to Sherlock's bosom, and they must deal with the slippery Irene Adler (Rachel McAdams), a woman who has scorned Holmes in the past and out-classed him intellectually.