MILD SPOILER AHEAD IF YOU DON'T KNOW THE MAIN CASTING FOR THE DOCTOR WHO CHRISTMAS SPECIALS

It seems the writers on The Simpsons might know the answer to the question on all Doctor Who fans' minds at the moment: the identity of the ominous figure who shall "knock four times".

In the latest show aired in the US, the annual Halloween special Treehouse Of Horror XX, there's a blink-and-you'll-miss it reference to the forthcoming Who episodes set to wrap up the Tenth Doctor era in spectacular fashion.

During the opening credits of the first segment, a black and white Hitchcock pastiche entitled Dial M For Murder Or Press # To Return To Main Menu, one of the Simpsons team is listed as 'DON "HE WILL KNOCK 4 TIMES" PAYNE'.

It's a clear nod to the unnerving warning given to the Tenth Doctor (David Tennant) by psychic Carmen in the Easter special Planet Of The Dead: "You 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."

So is Don Payne the villain responsible for the Doctor's regeneration this Christmas? Probably not, but it's certainly cool to see the allusion pop up in one of America's finest sitcom exports, now in its 21st season. Creator Matt Groening is an unashamed fan of Who and has made subtle references to it in the past, with a suitably bescarfed likeness of Fourth Doctor Tom Baker appearing in the background of several Simpsons episodes across its record-breaking twenty-year run.

As for the real identity of the guilty party, speculation is rife but the general consensus is that the "four knocks" relate to the resurrected return of the The Master, played by John Simm.