Knock, knock, knock, knock. Who's there.As David Tennant’s tenure as the Doctor winds down, clues to his demise are pouring in.

Back on the “Planet of the Ood” the Doctor received an ominous warning that his name would be remembered by “the Wind, the Ice and the Snow” but that his song would soon end. It’s no coincidence that the final episodes revolve around these same elements: “Planet of the Dead” – a storm of flying stingrays (wind), “The Waters of Mars” (water and presumably ice), and the as yet unnamed 2-part Christmas finale (snow).

And in last week’s Easter special “Planet of the Dead” a clairvoyant named Carmen also warned the Doctor to “be careful because your song is ending sir. It is returning. It is returning through the dark. And then Doctor… oh but then, he will knock four times.”

Naturally there’s a lot of speculation that “he” is the Master, which is supported by the fact that John Simm has been spotted on set. However this seems a little obvious, and besides didn’t that incarnation of the Master die?

First off, we have to realize Carmen said “it” is returning but that “he” will knock, two separate entities. It’s also important to listen not just to what she said, but how she said it. While the “it” is clearly connected to the Doctor’s song ending, the “he” if more of an afterthought and therefore we can’t assume “he” is evil. For all we know “he” could be the comical Meddling Monk, coincidentally it’s been suggested by some fans that the Meddling Monk is an earlier incarnation of the Master. And what would be more comical than the Monk knocking on the TARDIS and the Doctor responding “Who’s there?” Regardless of who “he” is, expect the Doctor to be paranoid about every knock and every tap.

Omega - Time LordAs for the “it” all we know is that it’s currently in the dark. One theory is that dark just means dead and that “it” is Gallifrey, the homeworld of the Time Lords, which was destroyed in the Time War. But why would that end the Doctor’s song unless this is another “Trial of a Time Lord.” On the other hand, dark could mean just that, which gives us three prime suspects: the Toclafane (which exist at “the dark and the cold” end of the Universe), the Beast (which was trapped in the event horizon of a black hole and later sucked in), and last but certainly not least Omega (the original Time Lord who was also sucked into a black hole while inventing time travel). All of which could easily be seen as an “it” by Carmen, especially Omega in his creepy costume (picture right). I’m routing for Omega even though that’s not likely since he hasn’t been seen since “Arc of Infinity” (1983).