Parts: 1 (60-minute)
Writer: Steven Moffat
Airdate: 25 December 2012
Production Code: n/a

Doctor: eleventh
Companions: Clara

Megalomaniac of the Week: Great Intelligence
Special Guests: Madame Vastra

Story: London, 1842, winter: reclusive young Walter Simeon prefers to keep his own company, rather than suffer the presence of other children, whom he regards with contempt. Having friends is unimportant, and he's not bothered that he doesn't even have anyone to talk with... until the day that his snowman talks back.

London, 1892, shortly before Christmas: Dr Walter Simeon, head of a notorious yet mysterious institute, is the only one aware that a new kind of snow is falling upon England. Psychic snow, able to respond to thoughts, able to remember... and able to feed on human flesh. As fanged snowmen begin to rise up around London, Madame Vastra — former Silurian warrior, now Victorian detective and huntress — realizes that the magnitude of the problem requires the Doctor's help. Unfortunately for her, and for all of humanity, the Doctor is not feeling charitable toward the universe, and is disinclined to interfere. Unless Madame Vastra and her cohort can appeal to his better nature, with the help of inquisitive governess (and part-time barmaid) Clara, the Earth may be blanketed in a permanent winter within days...