Dr Who supremo Steven Moffat has taken extreme care in protecting the plot of the programme's 50th anniversary episode to keep the interest level high prior to the episode's broadcasting in November.
To ensure the plot remains a secret, Dr Who lead writer Moffat has avoided writing the plot of the episode on paper, saying, "One length I've gone to which is a really good security measure- I make sure I don't get a script, because I will lose it. I forbid people to hand me one. It's on my computer under lock and key."
English actor Matt Smith, who plays the eleventh incarnation of the Doctor in the series, said, "I read it and I clapped at the end. I think it's hilarious, it's epic and it's vast. It pays homage to everything."
Smith, who was speaking at a Doctor Who series launch, said that fans will be intrigued and "will not be disappointed" by the special episode.
He also said he cultivated the art of being "discreet" in the sets of Dr Who.
"You're sat on all this information and people are genuinely intrigued. It's one of the responsibilities of being in this show- you have to be discreet about what you tell people."
Regarding the length of the special episode, an unnamed BBC source said, "The episode has been written but we don't know whether it'll be an hour, 75 minutes or 90 minutes long- That's in Steven's hand and he has free reign."
The episode is rumoured to aired in November in 3D to coincide with the show's birthday. The first Doctor Who episode was broadcast on 23 November 1963.
The Doctor Who series for this year will start on Easter Sunday, 30 March. The first episode for this year, Doctor Who: The Bells of Saint John, will introduce Timelord's new assistant, Clara Oswald (played by Jenna Louise Coleman).