The Doctor Who Christmas special, "The Time of the Doctor," will cap off a great year for the sci-fi series but the event will be a bittersweet one. Matt Smith's run as the 11th Doctor will come to an end and the new trailer for the season finale reveals he'll be facing plenty of familiar foes.

The trailer released by the BBC shows off Cybermen, Daleks, the Silence, Weeping Angels and a new character, Tasha Lem. Played by Orla Brady, little is known about Lem except that she's someone from the Doctor's past; and the actress has yet to reveal anything about the character in recent interviews. In the trailer, Lem gets a few lines and fans can see glimpses of the character in action, but her role in the "siege of Trenzalore" has yet to be revealed. The odds aren't in the Doctor's favor with mentions of a trap being set and Clara, his companion, pleading for help from an off screen character.

There appears to be plenty of action during "The Time of the Doctor" and the Daleks are keen on stopping the Doctor's regeneration. For fans, there are a few questions, such as the explosion of the Tardis in season 5, that have been unanswered and hopefully "The Time of the Doctor" serves as a way to tie up any loose ends.

Judging by comments by showrunner Steven Moffat, the Doctor Who Christmas special will serve as a way to properly end Smith's time as the Doctor before introducing Peter Capaldi as the 12th Doctor.

Well, Capaldi would have been the 12th Doctor had it not been for Moffat. Speaking to Mike Jeffries, Moffat has altered the numbering, with John Hurt serving as the Ninth Doctor which means Christopher Eccleston is now the 10th Doctor, David Tennant as the 11th Doctor and Smith as the 12th. That would mean Capaldi would be the 13th, but there was one more plot twist as Moffat said Tennant's Doctor used two regenerations, making Smith the 13th Doctor.

The numbering is important if Moffat is following established "Doctor Who" mythology stating there can only be 12 regenerations. "The Time of the Doctor" has to figure a way around this rule in order to introduce Capaldi as the new Doctor.

"The Time of the Doctor" will air on Dec. 25, on BBC One at 7:30 p.m. local time in the UK and on BBC America at 9 p.m. EST, 8 p.m. CST. The Doctor Who Magazine has a few hints about the Christmas special from Moffat, reports DoctorWhoTV. Expect some comedy, with the Doctor helping Clara cook, as well as a "fair bit of nudity."