A Scandal in Belgravia (S02E01)

A case of blackmail threatens to topple the monarchy itself, but soon Sherlock and John discover there is even more to it than that. They find themselves battling international terrorism, rogue CIA agents and a secret conspiracy involving the British government. But this case will cast a longer shadow on their lives than they could ever imagine, as the great detective begins a long duel of wits with an antagonist as cold and ruthless and brilliant as himself: Irene Adler. (BBC)