"The Snowmen" is the 2012 Doctor Who Christmas special, starring Matt Smith as The Eleventh Doctor, Jenna Louise-Coleman as Clara Oswin Oswald and featuring Richard Grant and the voice of Ian McKellan. Unlike other Doctor Who Christmas specials, it was aired in the middle of the series, rather than between series. Because of this, it is tied much more into the series arc than any other Christmas special, besides perhaps The End of Time.
After being introduced to the monster of the week, the titular Snowmen, we are reintroduced to the Doctor, who, since the events of The Angels Take Manhattan, has gone into retirement. He cynically refuses to help the world, despite the pleas of Madame Vastra and Jenny (last seen in A Good Man Goes to War). However, when Clara gets involved with both The Doctor and the Snowmen, he is drawn back into adventuring. What follows is a bit of slapstick, mixed in with running, some emotional twists and turns, and a scientifically shaky conclusion. Clara Oswald appears to die, but The Doctor realizes that he has already met someone of her name, and with her voice: the Oswin Oswald who died back in Asylum of the Daleks. Reinvigorated by both the personality of Clara and her mystery, The Doctor comes out of retirement to solve that mystery, which looks like it will be the important arc of the rest of the series.
Although this is a mid-series episode, it relaunches many things. The Doctor has had a character shift, and also a change in costuming. It also features a new introduction, a new rearrangement of the theme, and a redesigned TARDIS interior.
It is a good episode in many ways. I was most impressed by how Matt Smith could communicate how old and defeated The Doctor is: I don't know if any Doctor since William Hartnell's Doctor has been quite so abrasive and old. The chemistry between Matt Smith and Jenna Louise-Coleman is great, and the mystery looks to be intriguing. There are certain parts of the "monster of the week" aspects of the story that are predictably silly, but that is part of the bargain when watching Doctor Who.