The Abominable Bride

Let's start with the look of the thing. The Abominable Bride takes the modern cast, dresses them in the late 1890s and puts them back in a Baker Street of steam trains, hansom cabs, top hats and frock coats, with gaslight doing the work that CCTV usually does. It is a Christmas special with its priorities right: it wants to enjoy the trappings of the original stories and still hand Holmes and Watson a proper mystery.

The mystery is properly spectral. A ghostly bride prowls the streets with an unslakable thirst for revenge, and the city is troubled by her return. For a man who built his name on deduction, a ghost is a problem of a different order, and the episode has a quiet time playing Holmes's methods against a case that looks supernatural. Even Holmes seems to accept the game, trading his usual certainty for genuine unease.

Mark Gatiss, Steven Moffat and Arthur Conan Doyle share the writing credit, which tells you the mood, and Douglas Mackinnon directs. The whole company gets to dress up. Benedict Cumberbatch and Martin Freeman are Holmes and Watson, Una Stubbs is Mrs Hudson, Rupert Graves is Inspector Lestrade, Mark Gatiss plays Mycroft and Andrew Scott is Moriarty, with Louise Brealey's Hooper, Amanda Abbington's Mary, Jonathan Aris's Anderson and Yasmine Akram's Janine Donlevy filling out the gaslit streets.

The puzzle matters less than the pleasure of the conceit. And the conceit is a love letter to the Victorian originals from a show that usually cannot stop looking at its phone. It went out on New Year's Day 2016, which feels right for something this fond and this theatrical. It remains one of the most re-watchable hours the series has produced, a holiday treat that knows exactly what it is and doesn't apologise for a minute of it.