What made David Tennant's final days as "Doctor Who" so compelling was that he knew an end was coming and he simply didn't want to go.

But, it looks like that piece of drama -- as far as The Doctor is concerned overall -- is being removed from the series for good.

BBC revealed that Matt Smith's appearance in "The Sarah Jane Adventures" will declare the Time Lord as immortal and remove the 12 regeneration limit that was first introduced in "The Deadly Assassin" in 1976. The decision, of course, opens the door for the show to continue long after Smith regenerates into the 12th incarnation of the adventure-loving wanderer, and places him on a level playing field with his former companion, Capt. Jack Harkness (John Barrowman).

The biggest surprise about the announcement isnt that the Beeb has retconned that piece of mythology. Instead, it's the fact that it will not be handled in the series itself, forcing viewers to turn their attention to the spinoff with Elisabeth Sladen, directed at a younger audience.

An explanation for The Doctor's life-expectancy will be given in "Death of the Doctor," a two-part story written by former "Doctor Who" showrunner Russell T. Davies.

In the episode, The Doctor and Clyde Langer, played by Daniel Anthony, are in the process of "outwitting spooky vulture undertakers the Shansheeth, [when] Clyde asks how many times he can regenerate. The Doctor indicates that there is no limit. The action continues."

The 2010 Christmas special has already completed filming and will not only be a standalone outing, it also will act as a precursor to some of the events that will take place in the sixth season. Among the storylines touched on in the festive adventure is the "silence," which was hinted at in the final episode last season.

The episode will air in the United Kingdom on Christmas Day, and in North America before the end of the year.