Which Sherlock episode introduces Irene Adler in the BBC series?

“A Scandal in Belgravia” introduces Irene Adler in the BBC series Sherlock.

The episode is the first instalment of the second series and adapts elements from Arthur Conan Doyle’s short story “A Scandal in Bohemia.” Lara Pulver plays Irene Adler, a professional dominatrix who possesses compromising photographs connected to a member of the British royal family.

Sherlock is drawn into a contest of intelligence with Adler, whose ability to control information makes her a formidable opponent. Their relationship is presented through deduction, rivalry, attraction, and mutual recognition of each other’s capabilities.

The episode was written by Steven Moffat and directed by Paul McGuigan. It originally aired on BBC One on 1 January 2012. The title references Doyle’s story, but the modern plot replaces the original royal scandal with encrypted digital information and international political pressure.