His Last Vow

Charles Augustus Magnussen is the one man Sherlock Holmes truly hates, and the episode introduces him through a case of stolen letters. Magnussen is a media tycoon and a notorious blackmailer, and his trade is knowledge: he knows the personal weakness of every eminent person in the western world, and he keeps that knowledge filed and ready. He doesn't rob banks. He owns the people who do.

Holmes goes up against him knowing the fight is different in kind. Magnussen doesn't need smoke-filled rooms or hired guns; his weapon is exposure, and he is willing to be loathed as long as he is feared. The confrontation is less a case than a siege, and it tests Holmes in ways the usual villains can't. The show's villains usually want something, money or proof of genius. Magnussen wants nothing, which is what makes him frightening. He blackmails for sport and convenience, and there is no endgame to bargain with, which is a harder opponent for a detective whose whole method is predicting what people want. And the episode gets personal in the way the show's best outings do, dragging the people Holmes loves into the line of fire.

Steven Moffat, Mark Gatiss and Arthur Conan Doyle wrote it, Nick Hurran directed. Benedict Cumberbatch and Martin Freeman carry it, with Una Stubbs's Mrs Hudson, Rupert Graves's DI Lestrade, Mark Gatiss's Mycroft, Andrew Scott's Moriarty, Louise Brealey's Molly, Amanda Abbington's Mary, Jonathan Aris's Anderson and Lindsay Duncan's Lady Smallwood around them.

The episode is the darkest thing the show had done to that point, and it earns its reputation. What stays with you is the villain: a man who has never needed to raise his voice, because he already knows everything you'd rather he didn't. Holmes beats him, and the cost of doing it is the point.