In 1891, Irene Adler (Rachel McAdams) delivers a package to Dr. Hoffmanstahl—payment for a letter he was to deliver. Hoffmanstahl opens the package, triggering a hidden bomb which is prevented from detonating by the intervention of Sherlock Holmes (Robert Downey, Jr.). Holmes takes the letter and disposes of the bomb while Adler and Hoffmanstahl escape. Holmes later finds Hoffmanstahl assassinated. Adler meets with Professor Moriarty (Jared Harris) to explain the events, but Moriarty poisons her—deeming her position compromised by her love for Holmes.
Sometime later, Dr. Watson (Jude Law) arrives at 221B Baker Street, where Holmes discloses that he is investigating a series of seemingly unrelated murders, terrorist attacks, and business acquisitions, which he has connected to Moriarty. Holmes meets with the Gypsy fortune-teller Simza (Noomi Rapace), the intended recipient of the letter he had taken from Adler, sent by her brother Rene. Holmes defeats an assassin sent to kill Simza, but she flees before Holmes can interrogate her. After the wedding of Watson and Mary Morstan (Kelly Reilly), Holmes meets Moriarty for the first time. Moriarty informs Holmes that he murdered Adler and will kill Watson and Mary if Holmes' interference continues.
Moriarty's men attack Watson and Mary on a train to their honeymoon. Holmes, having followed the pair for protection, throws Mary from the train into a river below where she is picked up by Holmes' waiting brother Mycroft (Stephen Fry). After defeating Moriarty's men, Holmes and Watson travel to Paris to locate Simza. When she is found, Holmes tells Simza that she has been targeted because Rene is working for Moriarty, and may have told her about his plans. Simza takes the pair to the headquarters of an anarchist group to which she and Rene had formerly belonged. They learn that the anarchists have been forced to plant bombs for Moriarty.
The above taken from Wikipedia.
The story may start slow but it picks up in the middle and ends with the usual Holmes bang! LOL! Interesting, intellectual and enthralling, just loved the way Robert Downey Jr played the quirky Sherlock and Jude Law played his sidekick, Dr Watson. A match made in heaven indeed! There's never a dull moment with these 2 ^.^