Sherlock Holmes (Robert Downey, Jr.) is investigating a seemingly unrelated series of crimes around Europe, believing them all connected to Professor Moriarty (Jared Harris), a criminal mastermind just as smart as Holmes. After Moriarty arranges for another assassination, he poisons Irene Adler (Rachel McAdams), as her feelings for Holmes have compromised her usefulness. Meanwhile, Holmes takes Dr. Watson (Jude Law) out with his brother Mycroft (Stephen Fry) for Watson's stag party, and saves another intended victim of Moriarty's, a fortune telling gypsy named Sim (Noomi Rapace). Holmes meets with Moriarty, who warns Holmes that if he persists in investigating him, Watson will become a target. Holmes stows away on the train taking Watson and his new wife Mary (Kelly Reilly) to their honeymoon destination, knocking Mary off the train to the safe hands of Mycroft while he and Watson battle Moriarty's men. When the duo arrive in France, Holmes tells Sim that Moriarty targeted her due to her brother Rene's work with him, and she was a loose end.