The Christmas special is following Moffat’s tradition of adapting Christmassy stories, and this time it’s The Snowman. As with the previous two Christmas specials, it won’t be a truly faithful adaptation, and obviously has a science-fiction twist to it. The Snowman (or possibly Snowmen) isn’t/aren’t actually made out of regular snow. They’re made out of alien snow. Living snow with its own consciousness. And the snow is very, very hungry. Packed together into a humanoid figure, it’s deadly. Also, Vastra is narrating the episode.