Media playback is unsupported on your device Media caption Actor Matt Smith talks about how modern Doctor Who's Aliens are.

Doctor Who boss Steven Moffat is taking special care to protect the secrets of the programme's 50th anniversary story.

"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," said Moffat, the show's lead writer.

"I forbid people to hand me one. It's on my computer under lock and key."

Actor Matt Smith promised fans they "will not be disappointed" by the story.

"I read it and I clapped at the end. I think it's hilarious, it's epic and it's vast," he said at a Doctor Who series launch last week.

"It manages to pay homage to everything - and look forward."

The first story of Doctor Who's 2013 run, The Bells of Saint John, will be screened on Easter Saturday, 30 March.

Described by Moffat as a "proper London thriller", it sees the Doctor and new companion Clara (Jenna-Louise Coleman) battling an evil entity in the world's wifi networks.

Future episodes see the return of the Cybermen and old enemy the Ice Warriors, who last appeared during the Jon Pertwee era in 1974.

Image caption Matt Smith as The Doctor in The Bells of Saint John

"It's going to be the biggest and best and most inventive and most exciting year for the show," Matt Smith said.

The 50th anniversary special, due to begin filming in April, will be broadcast in 3D around the show's birthday in November.

Matt Smith told the BBC News website that during his time as the Doctor he had cultivated the habit of giving nothing away.

"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."

The first ever episode of Doctor Who, An Unearthly Child, with William Hartnell as the Doctor, was broadcast on 23 November 1963.

As part of the anniversary events, the BBC will also broadcast An Adventure in Space and Time - a one-off drama looking at how the sci-fi show came to be made.