Matt Smith will step down from his role as the current incarnation of the ever-changing protagonist of "Doctor Who" on Christmas, with Peter Capaldi taking over at the Twelfth Doctor. The episode, "The Time of the Doctor," will air December 25th at 9pm on BBC America and will find the Doctor sacrificing everything to save a town called Christmas from his greatest enemies, including the Cybermen, Silence, Daleks, and Weeping Angels. Check out a teaser for the special below: