The first ever broadcast of the trailer was accompanied by a short minisode prequel, set some time between the deaths of Rory and Amy Pond, and the chronology of the Christmas episode. The Doctor is in nineteenth century England with a Silurian, a Sontaran and a human woman (sounds like the start of a bad joke). There is also mention of a meteor shower in the minisode, and later in the trailer, the Doctor talks about mind-controlling snow. Do I see a plot developing here?
The minisode shows the Doctor declaring that he has retired, one assumes due to the trauma of losing Rory and Amy to the Angels, and the effect this has had on his ever-active conscience. He also looks very handsome in Victorian getup, I might add.
In the trailer we see Jenna-Louise Coleman as Clara, a nineteenth century young woman, who Matt Smith and Coleman hint in the commentary might be the character to bring the Doctor out of retirement. It's not made entirely clear whether Coleman is playing the same character from the last series, Oswin. According to IMDb, Oswin's first name is Clara, which leads to questions about Oswin's demise and transformation into a Dalek. I'm sure Moffat will answer all questions when the latest season resumes.
The first few peeks at the Christmas Special and upcoming episodes have so far been promising, with typical Who fodder and Moffat writing coupled with a fresh face in the form of Coleman. So far she's shaping up to be a very promising new assistant for Matt Smith and the Doctor Who fandom.
