Doctor Who turns 50 today. Huzzah! Matt Smith and some of the previous Doctors.

This Saturday the 50th anniversary special episode of "Doctor Who" aired worldwide, and fans were enthralled to see something that's rarely happened in the show's mythology: Multiple incarnations of the Doctor working together (plus some slimy aliens trying to take over the planet, but ho hum to that). But this year's Christmas special promises to be even more shocking.

Spoilers for said Christmas special lie ahead. See, it's a major tenet of Who mythology that the Doctor can only regenerate 12 times. Matt Smith's Doctor, who the audience thought was No. 11, has totted it up and realized he's actually the 13th, and last, incarnation. So how does he manage to regenerate into "The Thick of It" star Peter Capaldi? Don't worry, "Doctor Who" has no plans to gloss over this very important problem.

"The riddle of the regeneration problem, something fans have talked about for decades, will be faced head on at Christmas," a source close to the "Doctor Who" production team told the Daily Mirror. "There is going to be another huge cliffhanger, and somehow Peter Capaldi has to join and the series has to continue."


The source added, "The show's big fans, known as Whovians, won't believe their eyes at Christmas."

It's still a sci-fi show that uses a magic police box to travel through time and space, right? I think my eyes will probably be fine. They are pretty used to CGI and abrupt rewriting of sci-fi backstory.