It sees this pair back in late 19th century London (with no explanation apparently), trying to figure out the truth behind an eerie ghost story. That all sounds terribly cinematic, so it's great to hear that the episode will getting a big screen release in cinemas.
The 90 minute special is coming to cinemas around the world, including opening in China. The BBC is calling it a 'global cinema event' which should give you some idea of just how popular Sherlock is. Of course, it's main stars Benedict Cumberbatch and Martin Freeman are no strangers to that bigger screen, with credits including The Hobbit trilogy, Black Mass and many more between them.
The episode sits apart from the man show, which should be a good excuse to do some fun things with the formula. We've already heard that Holmes will be a bit more formal and gentlemanly; his manners in the modern era sometimes aren't the best. Rupert Graves, Una Stubbs, Louise Brealey and Amanda Abbington will also be along for the ride, with a script written by regulars Steven Moffat and Mark Gatiss.