t is returning, it is returning through the dark ... he will knock four times." The psychic lady on the 200 bus said these darkly prophetic words to The Doctor during the conclusion of 'Planet Of The Dead', whipping up the tension ahead of David Tennant's impending departure. But who exactly wants to get in touch with The Doctor? We look at some of the possible characters who might be flexing their knuckles (or tentacles) in advance of the knocking...
The Face of Boe
Although we've seen the bodyless Face die on screen in 'Gridlock' 5 billion years into the future, he's still knocking around until that time and has been closely associated with cryptic messages in the past. "Know this, Time Lord - you are not alone," he memorably warned The Doctor. Perhaps there's something else Boe needs to tell him? However, given his lack of limbs the act of knocking may prove to an insurmountable obstacle ... unless Boe headbutts the Tardis door four times. Now that would be fun to watch!
Set reports have indicated that the benevolent squidface will pop up during 'The Waters Of Mars', and there's every chance that he'll announce that The Doctor's song is coming to an end. He made a similar prophetic proclamation at the end of the brilliant 'Planet Of The Ood', after hailing 'Doctor-Donna' for their help in helping the Ood to break free from their enslavement. Could Sigma play the same role in the Tenth Doctor's regeneration as The Watcher did for the Fourth Doctor's transition?
Maybe the mutant mentalist had one more Emergency Temporal Shift left at the end of 'The Stolen Earth'? The stresses of surviving the Time War screwed up his mind, but he ultimately helped to thwart Davros and defeat the hordes of Daleks poised to conquer the universe. Caan could well want to catch up with the Time Lord for a pint and gain some much-deserved recognition for his selfless actions. But with those moist, flaccid tentacles, could anyone actually hear the poor blighter knocking?
The Grim Reaper
David Tennant recently told the Radio Times: "The end is nigh. You can sense the Grim Reaper knocking on the door. But how many times?" Well, quite possibly four! If you think that the physical embodiment of the Grim Reaper is something that happens in Scrooged and not the Whoniverse, then check out the superb Torchwood episode 'Dead Man Walking', when Death itself pays a CGI-aided visit to Cardiff. Could it be returning through the dark to claim a certain Time Lord?
It has been strongly suggested in the series that Jack is actually The Face Of Boe, but in the meantime the Torchwood lothario is loitering around Cardiff dealing with Weevils and Gwen Cooper's PMT. He memorably came calling for The Doctor towards the end of the third season in 'Utopia', so there's every chance he wants to check out the latest Tardis talent. Sadly, The Doctor is now without a companion so his libido may well be disappointed.
He'll huff, and he'll puff, and he'll knock your house down - four times! Okay, the tale of Three Little Pigs doesn't quite go along those lines, but Rose Tyler's wordy alter ego surely deserves its own physical manifestation by now. Given that the country is in the grips of a recession, the BBC should dig out the prop used for The Garm in the 1983 story 'Terminus', give it to Tim Westwood and his crew for a quick pimp up, and unleash it as the Bad Wolf for Tennant's farewell.
Donna Noble's grandfather knows the secret of the Chiswick temp's memory erasure, and there's every chance that something goes wrong down the line. Bernard Cribbins is being given full companion status for the last two stories of David Tennant's tenure, so he's definitely in the frame (not Midshipman Frame we hasten to add!) Then again, perhaps the newspaper vendor short-changed The Doctor when he first encountered him in 'Voyage Of The Damned' and merely wants to return some pennies?
The Drowned Hath
The poor fish-faced creature who saved Martha Jones in 'The Doctor's Daughter', only for her to stand and watch as he slowly expired, might be a bit of a long shot for a return in the fin-ale. But perhaps the quicksand he fell into led to a black hole that took him back in time, which gave him another oppor-tuna-ty in life. Now he wants revenge for Martha's sole-destroying behaviour and The Doctor's realm is the best plaice to start looking. Or is that theory just codswallop?
Here come the drums, here come the drums. Okay, so it appears very likely that The Master is behind the impending knocks given his previous tendency to tap away, plus John Simm has been sighted on set during filming recently. Being killed has never stopped The Master returning for vengeance in the past, especially as the mystery of his ring is still waiting to be solved. Maybe though, this shady character's comeback is a little too obvious to be 'the knocker' and a red herring?