A Scandal in Belgravia

The government summons Sherlock Holmes. That's how it always starts when the trouble is too delicate for the police: compromising photographs exist, and if they reach the wrong hands they threaten the monarchy itself. The summons comes with the usual family strings attached, because nothing this delicate ever reaches the police. The job is simple to state and impossible to do, because the photographs belong to Irene Adler, and Irene Adler is not the kind of opponent who hands things over.

The episode sets Holmes and Watson into a contest of wits with national consequences, and the game is on in more ways than one. Adler is always two moves ahead, and she seems to enjoy the duel more than the outcome. The stakes could not be higher, but the tone stays playful, which is the right call: this is a story about two clever people enjoying each other's company, with a monarchy dangling in the background. Mycroft's presence raises the family stakes, because the brother who runs the country can't quite run this one.

Steven Moffat, Mark Gatiss and Arthur Conan Doyle wrote it, Paul McGuigan directed. Benedict Cumberbatch and Martin Freeman lead, with Una Stubbs's Mrs Hudson, Rupert Graves's DI Lestrade, Mark Gatiss's Mycroft, Andrew Scott's Moriarty and Louise Brealey's Molly Hooper around them, plus Danny Webb as DI Carter, Andrew Havill as the Equerry and Todd Boyce as Neilson.

It is the episode where the show finally meets a mind that can keep pace with Holmes, and it has the confidence to let her win the duel even when she loses the case. For once the most dangerous person in the room is not trying to destroy him. She's just trying to get the better of him, and she does. And Holmes will spend the rest of the series pretending it didn't matter.