The Hounds of Baskerville

Dartmoor is a good place for a horror story, and the episode knows it. Sherlock and John travel there because a young man called Henry Knight keeps seeing a monstrous hound on the moor where his father died. The creature, whatever it is, apparently killed Henry's father, and Henry has been carrying that sight with him ever since.

The investigation is a proper gothic outing, and the locals circle the mystery from every angle. DI Lestrade helps with the police side of things, Mycroft's interest suggests the case reaches further than Dartmoor, and the doctors, Dr Stapleton and Dr Frankland, offer explanations that don't quite hold. Even Major Barrymore has a part to play. Everyone has a theory, and the theories keep colliding.

The show is honest about the appeal of the supernatural here, and it makes Holmes work to refuse it. The tension between what the witnesses swear they saw and what deduction says happened is the whole episode, and it's the sort of story Conan Doyle wrote best: fear that lives in fog and folklore. The hound is the whole point of the genre, and the show knows better than to explain it away too quickly.

Mark Gatiss, Steven Moffat and Arthur Conan Doyle wrote it, Paul McGuigan directed. Benedict Cumberbatch and Martin Freeman are the leads, with Una Stubbs as Mrs Hudson, Rupert Graves as DI Lestrade, Mark Gatiss as Mycroft, Andrew Scott as Moriarty, Russell Tovey as Henry Knight, Amelia Bullmore as Dr Stapleton, Clive Mantle as Dr Frankland and Simon Paisley Day as Major Barrymore.

Russell Tovey's Henry is the heart of it: a man who knows what he saw and can't make anyone believe him. The episode is the series' most atmospheric hour, all fog and family grief and a monster that may or may not be real. And it's a reminder that Holmes's greatest enemy has never been a criminal. It's the world insisting on one thing while the evidence says another.