William James Moriarty and Sherlock Holmes meet again on a train back to London, and a dead body is discovered in a locked passenger cabin. With John Watson named as the prime suspect, Sherlock challenges Moriarty to a contest of who can solve the murder first. Will Sherlock be able to clear Watson's name? Back in London, a state secret that only the royal family is privy to has been stolen and MI6 is tasked with retrieving it, lest the whole empire fall into ruin! Meanwhile, Sherlock is visited by the king of Bohemia, who asks him to cover up a recent scandal..