This Saturday will see the last episode of the current series of Doctor Who on BBC One.

Last week's cliffhanger saw star David Tennant blasted by a Dalek, raising the possibility that he will leave the show, and sparking furious debate amongst fans about the finale.

BBC News entertainment correspondent Lizo Mzimba looks at how the show's plot twists are kept a secret - and the confidentiality agreements signed to ensure they are kept hidden.

BBC News' Lizo Mzimba in the Tardis

It's 1 May, 2008 and I'm standing in the Tardis, where I'm watching...

Er, sorry, I can't tell you any more, even if I wanted to, as I've signed a secrecy agreement promising not to reveal what I've seen until the appropriate time.

It's just one measure the Doctor Who team have taken to keep plot twists fresh for the audience.

Storytelling in television is often reliant on surprising the audience, something that is especially true for programmes like Doctor Who. And it's been part of TV for decades.

When it was revealed who'd shot JR in Dallas in the 1980s, the show was transmitted in the US before the UK.

And in a world before the internet, the secret was kept for the 21 million Brits who tuned in.

There is no way that would happen now.

Impatient fans of modern-day US hits like 24, Lost and Heroes set their hard disks whirring within minutes of transmission, downloading and discussing the latest plot twists.

The fourth series saw Catherine Tate as the Doctor's assistant

And with dozens of websites dedicated to the Gallifreyan Time Lord, it is a huge challenge to keep under wraps what will happen in the series finale of Doctor Who.

It's worth pointing out that the vast majority of the show's fans don't spend their time combing the internet for spoilers.

But when an episode ends like last week's with the Doctor seemingly mid-regeneration, the usual rules fly out the window.

I know of one family of kids who are usually content to wait for each episode, but last week they were so filled with excitement by the possible prospect of a new Doctor that they immediately started sweeping websites to find out what might be coming.

The word "might" is important here. There's a lot of information on the web. Some of it is possibly genuine, the majority is bunkum. And because the final episode of the series has been shrouded in secrecy, it is difficult to ascertain what might be true.

The wild speculation ranges from elaborate theories involving the Doctor's hand, to complex flashback scenarios - which even I have difficulty following.

So how have they managed to keep what happens in the final episode secret?

Well, for the first time this series, journalists haven't been sent a preview DVD.

The Daleks are the distillation of all that is evil in the Universe

Even so, there are hundreds of people working on the programme who could surreptitiously reveal what happens. And it's a tribute to their loyalty that they keep their mouths shut.

Yes, there are security measures like scripts marked with the name of the individual whom it's issued to, but that's never, ever going to stop a determined leaker.

Even when people sign secrecy agreements it would be impossible to track down who revealed a particular fact.

Bitter experience has made the Doctor Who team adept at keeping the audience guessing.

Billie Piper's unexpected appearance in the first episode of the series was achieved through a complicated game of subterfuge involving two different master tapes.

And, even now, a search of the darkest corners of the web only comes up with a few vaguely plausible sounding ideas for the finale.

That's just the way the Doctor Who team want it - ensuring that the biggest possible audience will be sitting in front of their TV sets on Saturday night enjoying the episode and its revelations at the same time as everyone else.

The final episode in the current series of Doctor Who will be shown at 1840BST on BBC One on Saturday 5 July.