The Reichenbach Fall

Moriarty is back, and this time he isn't robbing banks or setting bombs. He's doing something cleverer: he's going to destroy Sherlock Holmes's reputation and turn the public against him, using the most audacious crime of the century as the tool. The plan is an indirect attack, and it works better than any of his bombs, because it makes everyone wonder whether Holmes is all he claims to be. The weapon is a story, and the story is aimed at the one person in London who can't afford doubt.

The episode is a long, slow demolition. The newspapers turn, the questions get louder, and the people closest to Holmes are forced into uncomfortable positions as their belief in him is tested. Lestrade and Mrs Hudson hold on longer than most, and it costs them. Molly, Donovan and Anderson circle the wreckage in their own ways, and Mycroft watches from a distance with the look of a man who saw this coming.

Andrew Scott's Moriarty gets his finest hour here, dangerous precisely because he is having such a good time. Steve Thompson, Mark Gatiss and Steven Moffat wrote it, Toby Haynes directed, and the cast is the series' best company: Benedict Cumberbatch, Martin Freeman, Una Stubbs, Rupert Graves, Mark Gatiss, Andrew Scott, Louise Brealey as Molly Hooper, Katherine Parkinson as Kitty Riley, Vinette Robinson as Sgt Sally Donovan and Jonathan Aris as Anderson.

And it ends the way the title promises, on a rooftop, with the series' most famous fall. It is the rare hour of television that makes you want to watch the next episode and never forgive the last one. The question it leaves you with, the one that followed the show for two years, is whether the man on that roof was a hero or something far harder to name.